Experience: I have a degree in audio engineering. And it is my passion. =]
These guys are absolutely correct, a large diaphragm condenser is the way to go. Get a large diaphragm condenser with an XLR output, not USB or 1/8th inch or anything like that. And, as others have said, you'll need something to provide phantom power to the microphone, and something to act as an interface to your computer.
Microphones heavily fall into the area of diminishing returns. A $50 large diaphragm condenser is significantly better than a headset or lapel mic. A $200 large diaphragm condenser mic is better, but not "4x" better. An $800 version is better still, but the difference is subtle. And a $2,000 version is better still, but again, the difference is subtle.
Behringer, Nady, and MXL are all bargain basement brands. But they can get the job done. I'd say avoid Nady, but if you're on a budget, Behringer or MXL should work fine for your needs. If you have a bit bigger budget, the R0de NT1-A is a great mic for ~$230. Or one of the cheaper options from ADK could be great too.
By "isolation room", don't go crazy. You need a quiet environment, but closets don't sound good. And anyone who tells you to the contrary is misinformed (unless the only other option is a bathroom, for example). Closets can be a necessary evil if you are recording in a less than quiet environment, but large rooms sound better. If you live in a quiet house or apartment, for the love of donuts, don't record in the closet, and please don't start hanging egg-cartons or blankets on the walls (too thin to be effective, an eye-sore, and a fire hazard to boot). Turn off your AC, keep the microphone in a different room than your computer or anything with fans, and record when no one else is around to make noise.
If you're mildly serious about sounding good/professional, buy the Portable Vocal Booth made by Real Traps. It costs $300, and it provides 80% of the benefit of a a room fully treated with absorption. (Watch demonstrations on Youtube if you don't believe me, the effectiveness is startling.) Placing a large diaphragm condenser in the middle and speaking into it sounds surprisingly good (and much better than the competing brands of these type of products). If you're handy, you could build a product like this for less than $100 using Owens Corning 703 rigid fiberglass, a frame, some hinges, and some breathable cloth. A poor man's version of this would be to position some pillows or couch cushions as if they are the Portable Vocal Booth's walls. Thick pillows and and couch cushions that are fabric covered (not leather) are definitely the best. (And, of course, a rich man's version of the Portable Vocal Booth is to build a room without parallel surfaces with triple-thick sheetrock walls, and to cover >75% of the walls and ceiling with absorption and diffusion deep enough to be effective to low frequencies. If you spend more than $20,000 to go this route, I sincerely hope your fellow Skyrim modders appreciate it.
Something I don't believe the others mentioned (unless I missed it) is headphones for recording. You need something isolating, so the microphone doesn't pick up what you're hearing in your headphones. Earbuds and most over-the-ear headphones won't work well for this at all. If you have in-ear monitor headphones (that have dual or triple rubber flanges and look like ear plugs), that could work well. Or some large headphones that are closed-back or sealed, as you often see in studios. Most large headphones seem to be semi-closed-back and don't provide great isolation, so whatever you hear in your headphones while recording will be somewhat picked up by your microphone. Most of the world's best sounding headphones are open-backed and leak a substantial amount of noise. But you need closed-back headphones for recording. I've seen lots of people use the Sony MDR7506 for $80, and the Sennheiser HD 280 for $100 is popular too. Or, if you're on a budget like me, order the MoreMe headphones. They're butt-ugly and don't sound the best, but they are durable, very isolating, and dirt cheap ($22). Perfect for recording. Mine came in the mail yesterday.
Good luck. Drop me a line if you need any help picking out products or building absorption. And let us know when the mod is ready!