As much as I hate to say it, snag a Dell and drop in a sound card and video card. Dells have come a long way since they first got big. The quality of components has gone up and you just plain can't beat the prices.
Like fargoth said, get something with a dual core processor. Preferably one of the Core 2 Duo's... E6600 or E6700. If you can get one with a motherboard that allows upping the bus speed, you can overclock the E6600 and E6700's like mad. I've personally gone from 2.67GHz to 3.3GHz with an E6700 without having to modify the supplied voltage and with regular air cooling (a decent Zalman cooler though). Some people have hit 3.5GHz without any other mods except upping the bus freq. A good thing to look into with the motherboard would be one with an X38 Intel chipset as it'll be able to support the later generation Penryn processors when they come out... adding a bit of scalability. Another good thing to look out for would be for a motherboard with dual x16 PCI-e slots so you can slap in two video card and run them in SLI when the prices drop... especially if you plan on getting a big monitor and running high resolutions (which is where SLI really shines).
On the video card its self, go with an 8800GT like fargoth said. They just recently hit the market and let me tell you, they hit it like a freight train. The bang for the buck performance is phenomenal. Under half of the games and quite a few benchmarks they run neck and neck with (and sometimes beat) the $400 8800GTS's. The power requirements are also 10-20W lower along with less heat output due to NVidia shrinking down the die from 90 to 65nm. Your looking at around $250 for a 512MB 8800GT (A year ago I spent $580 on my 8800GTX!
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On ram, go with 2GB no doubt. Especially if your running Vista which is a memory hog with its pre-caching ability. For a gaming rig, 4GB would be optimal. If you were just running XP (wouldn't take advantage of the video cards DX10 ability though) you could get by easily with just 2GB.
For the hard drive get a 10,000RPM Western Digital Raptor. You have the option of a 74 or 150Gb model. You also would have the option of running a RAID setup (where the data is stored on both hard drives to speed up transfer rates) if you wanted to add another like hard drive later on.
On the sound card, get a cheap Creative XFI Xtreme Audio... especially if the computer comes with onboard audio. Onboard audio taxes the processor for work at a MUCH greater amount as opposed to a dedicated sound card (with it's own processor) freeing up your main processor to do other work.