When it comes to making high quality productions, mic quality is right up there with just plain being a good actor with what will get VAs cast or not. Simply, if the recording doesn't sound clean, it will sound unpleasant and be distracting in a project, regardless of your vocal skill.
There are various mics out there that people recommend as better/worse in quality, but a lot of them can get up to over $100. In short, replacing mics as a sole way to improve quality can be quite expensive.
Surprisingly, the best way to improve your sound quality if it has been a problem for you -- and most cost-efficient, for those of us with tight budgets -- usually is not your mic itself per se. I've been able to get very good sound quality, that people have personally remarked to me on, out of my Logitech USB Desktop mic (a decent multipurpose mic that goes for around $25), using some prudent set-ups and some cheap additional tools that are easy to put together.
The big things that lower recording quality are: pops/puffs, clipping, static, background noise, and reverb.
Pops and puffs are those loud 'PUH!' sounds that might happen when you say words with 'p' and 'h', where air is 'puffing' out of your mouth. You can make your own pop screen if this is a problem for your mic, but what I did to just about eliminate them from mine was to angle the mic about 30-45 degrees to the side when recording. Your voice goes in all directions but the air puffs go only in front of you, so that way the sound reaches the mic and your breathing doesn't. =D
Clipping (also known as peaking) is when your sound is too loud for your mic, for example when you shout. It distorts the sound so you hear this dull 'scree' sound across your recording. There is no good way to remedy this with sound editing, so if it happens you just have to record again. =x
The way to avoid this, though, is to lower your input level on your mic when you know you're going to be saying something very loudly.
If you have a PC:
Start menu -> Control Panel -> Sounds, Speech and Audio Devices -> Sounds and Audio Devices -> Audio -> Sound recording: Volume; lower this slider for yelling lines, raise it back up for normal lines.
I dunno how to do it for Macs (if someone *does* know, please do post it here!) but it should have a similar setting to tweak.
For iMac Clipping:
System Preferences -> Sound -> Input -> Then select whatever mic you are using and just shift the volume down. There is also a nifty ambient noise reduction that you can use if you want. (<-- thanks lightofhope18!!)
You could just leave the input setting at the lower end all the time, but then quieter lines will have to be amplified a lot more, and that will let any slight static in the recording be magnified (which happens with even the higher end mics). This doesn't matter for yelling because that's already a very strong input, so static will already be minimal.
Static is a bit different from background noise; static is a slight hiss you might hear if you amplify a quiet recording; it's just the ambient noise of the mic recording, period, and it has to either be taken care of by noise removal (which does lower sound quality a little), or by being judicious with sound input as said above for clipping.
If you're working next to your computer, a side note, your mic will pick up the sound of the computer running, and the closer it is, the more obvious it is. D= A trick to this is, get your mic farther away from your computer's buzzing -- but vertically. Putting it farther away horizontally actually isn't quite as effective; if you put it even like an inch or two above your computer's noise factory, it will cut down on it significantly, and the rest you can get rid of by the other methods listed here.
Background noise on the other hand is like a dog barking, construction work outside, paper rustling etc. Noise removal doesn't work so well on those, so like with clipping, you'll need to just rerecord if it happens. D=
Buuuuut, if it's mild enough, you could get rid of it the same way you can get rid of reverb --
Reverb is referring to the echoes your mic picks up when sound rebounds off the walls of your recording space. While slight amounts can be masked, for some scenes like those outside you really want to have as little as possible. Here, your mic isn't the issue: it's your recording space, and that's a little more tricky.
If you happen to have easy access to a walk-in closet with lots of clothes in it where you could record, the clothes and the small room act perfectly as a makeshift sound booth and I got *really* good recording quality from that.
If not (as I don't right now either), take a look at this:
Basically, the Porta-Booth will absorb the echo that your voice might otherwise generate before it ever gets out to the walls. This will work even in rooms that aren't the size of a walk-in closet, and you can put it right on your desk next to your computer and it works like a charm.
You can make your own Porta-Booth a lot cheaper than buying it pre-made ($100+ pre-made is totally ripping you off XP ): get one of those fabric storage cubes you might use for a dorm room and line the inside with 2-inch pyramid acoustic foam as it says at that website; you won't even have to attach them, they should be cozy enough to stick in the cube by their own weight.
Here's where I bought my acoustic foam at a great price:
The 12-pack is the cheapest I can find anywhere online even with shipping costs, and I would love to hear from anyone that can find something better (no I don't work for them, I bought a pack for myself and am already using it, so I can affirm these guys are legit). The total for a homemade Porta-Booth including the price of the storage cube comes out to around $45 or less: much less than what buying a new mic would likely need. =P (Actually, the above I think would be helpful even for those with high end mics, on how to best bring out the quality that those kind of mics can provide.)
If you're looking for some more advice, this thread also has general information on mic quality enhancing as well as other techniques, especially if you're still new to VAing:
I should make an addendum, if you're already quite advanced with your VAing technicals and are trying to improve it from something already quite good, then that's where buying a new mic would start doing something for you -- it's the subtle things that new mics improve, not eliminating stuff like clipping and reverb, which mics really can't do much about by themselves.
Any other questions or comments about the above, feel welcome to speak up! ^^