That depends what you need with it... the big one is the monitor. If you need a new monitor, and you want a nice gaming monitor, subtract at least $200 from that budget, and more likely in the $300 range. More if you want a big flat panel.
However, for about $2000 we can still put together a pretty sweet rig.
So to begin with, we look at CPUs. With that money, I'd spring for an Athlon 64. Most things won't take advantage of it yet, but it will be worth it in the long run. So we'll say an Athlon 64 3000 for about $200.
I don't think you'll want the stock heatsink that comes with some retail packages, so you'll need another heatsink/fan unit. I don't know enough about these to recommend a good one (I think Thermaltake is pretty good), but we'll say about $30 for the unit plus a little thermal grease.
We'll need a motherboard to go with it, though which one is not crucial... so you can get a solid MSI or Gigabyte for about $80. Try
this.
For RAM you'll want a pair of 512s (for a total of 1 GB), probably, since that's going to run fastest with the 64. If you're going to do overclocking at all, you'll need high quality RAM, which is unfortunately expensive. So you can get 2x Kingston HyperX (their "gaming" RAM) with built in heatsinks for about $300.
Here.
Video Card - Probably the most important part of a gaming machine. Since you have the money, you may as well get the best. I prefer ATI, so I'm going to recommend the Radeon X800 Pro,
for $400.
Sound Card - Pretty important. If you want to watch DVDs or have surround sound, you'll need a good one for a good sound experience. A Sound Blaster Audigy 2 ZS for $90,
here.
Hard drives- I'd toss two in, just because you can. Western Digital 36 GB 10,000 RPM
Raptor $110
Then any 80 GB 7200 RPM drive ~ $80.
Let's see. So we have the expensive stuff out of the way. If I added right, we're at about $1600 right now.
So all you really need to add is external drives (DVD-RW, CD-RW, whatever you want... $150 or so), a case with a good PSU ($150 tops. Unless you want mods
), buncha fans and cables and whatnot ($100 maybe), an OS and some shiny new virus software ($150ish) and you're about set.
So to recap.
Athlon 64 3000 ~ $200
Heatsink/Fan unit ~ $30
Motherboard ~ $80
2x 512 MB pc 3200 Kingston HyperX ~ $300
Radeon X800 Pro ~ $400
Sound Blaster Audigy 2 ZS ~ $90
Western Digital 10,00 RPM 36 GB Raptor ~ $ 110
80 GB 7200 RPM HDD ~ $80
Random stuff I mentioned at the end ~ $550
Estimate: $1840.
These are all rough prices, because I'm not sure where you want to buy all your stuff. I also don't know how "hardcore" you want it, so I just took a guess. Furthermore, there's about a 93.6% chance that I've forgotten something (too late... must finish post...) that will add another couple hundred to round out the $2000.
Then you can get a sweet monitor and some pimpin speakers, and you'll be set.
Edit: I started writing this response before either of these other two hooligans. Listen to me first