AMD 64's are the creme-de-la-creme of gaming PC's but you need to double your budget first.
If you get a copy of my system you will be doing very well for yourself, though I have heard that since ATI and Valve have been in bed on this from the getgo, it'll run better on an ATI card.
Which isn't a bad thing! ;b Let's see what that would come to...
MOBO: Asus P4C800-E Deluxe - $175
CPU: Pentium 4, 2.8 or 3.0 Socket 478 - $180
RAM: Corsair TwinX 2X256MB - $125
Video: ATI Radeon 9800 Pro 256-bit, 128MB - $200
Any CD-ROM's, add $30 each; a good CD-RW/DVD should cost no more than $70 with Nero Express.
You have the HDD and the sound solution, so that totals only $680!
So with the change, buy a really nice Thermaltake Xaser or Daimer case, and maybe a DVD burner to flesh the PC out.
Just make sure the case DOES NOT come with a PSU...get a good Antec 480w TruePower unit for around $75. This is not an area where you want to skimp, trust me.
But since that comes to $750 or so already for just the base of your platform, if you want to go whole hog and figure in an additional $40(CD-RW), $70(DVD-+RW), plus $120 or so (Cool Case), that's going to go over your budget by about $200....w/o figuring in the OS. Do you have a copy of XP handy? If so, that'll save you over $100 right there.
This will, however, give you one heck of a gaming machine.
And I really can't recommend cutting corners anywhere. Quality costs; you might look for a $50 case, but remember that the PSU is the MOST important part of any PC; do not buy a cheap one.
If the budgetary concerns are serious factors, then try getting the rig as shown with just a CD-ROM, then add more RAM modules and buy the CD-RW or DVD burner later.
Good luck, and have fun!