I built my latest machine for around $1,000 about three months ago. You could build the same PC now for even less, assuming you already have a monitor. I wish I had 2k to put into mine, but for what I spent I am quite happy.
Intel Pentium 4 "Prescott" 2.8Ghz, 1MB Cache, 800Mhz Bus
Abit IS7 Motherboard, 800mhz Bus, Serial ATA, Dual Channel RAM support, all toys..
2x512MB DDR400 RAM, running in dual channel mode (1GB total).
Radeon 9800 Pro 128MB DDR video card, 256-bit pipeline.
Lite-On CD-RW/DVD-ROM combo drive.
Seagate Barracuda 80GB Serial ATA Hard Drive
Raidmax Dragon Case, x5 80mm fans, lots of LED's, clear side panel.
It plays Doom III with amazing performance (with the ATI beta drivers), and just about every other game I've tried with awesome fluidity. I scored over 100 average frames per second on the Half-Life 2 stress test. I kind of regret not buying an x800 card for my machine, but when I built it they were very scarce anyway. With $2000 you could build a beast of a machine..