Heres your first person telling you to go Intel. Get a Conroe E6600 Dual Core Processor, it may be similar straight out the box to the AMD 6000 X2,but the overclocking potential is Huge, You could easily make it as fast as some of Intels Quad Cores at stock i would think,(900USD). Also swap that GTS to a GTX, and Even go SLI if need be. Swap the motherboard for a ASUS P5B Deluxe.
Then your system will be Top notch, BUT!, I have a better reccomendation... Hold back for the ATI X2900XTX real benchmarks are released, so we can see wether the upgrade to a X2900XTX would be substantial, and worth the wait.
I would change the ram to some PC6400 Crucial Ballistix, but again, thats just my preferance. Maybe another 2GB Would be nice, but then you'll probably go over budget. 4GB of ram is not needed at this moment in time by any means for gaming, but will hold out well for gaming.
Personally i wouldnt be to fussed over getting a SATA or IDE Harddrive, SATA are slightly better, but dont get all bothered about it. 500GB for me, would be to much, a 300GB would be plenty for me, That might be something you might like to take into consideration. Also good brands are Western Digital, Seagate, and Hitatchi in my opinion, try and make sure it has atleast 8MB Buffer and 5400RPM.
Another point is, that you probably wont need VGA Cooling. Find a Normal GTX Catd, then look for a 50 dollar more card, you'll probably find its got built in better-than-stock cooling.
You'll probably want atleast a 600W PSU to Drive this BEAST of a set up, but a 700W+ PSU to be safe (DONT be drawn in by the 1200W ones, there RIDICULOUS) Brands to look out for (good ones) are...Hiper, NorthQ,Seasonic,Coolermaster, and Tagan are brands that i would reccomend for being
Another nice thing to have is a Ageia Physx card, certainly not needed, but a few games are starting to be released now that support Physx cards.
The X-Fi ExtremeGamer soundcard is good, but many will argue the the XtremeMusic is a better card. If you want more opinions on this, go into User Control panel (User Cp) and send a personal message to either Crysalis, or EricB. They'll be happy to help, or simply post is the Audio and Multimedia section on tech forums.
If you do order a Intel CPU, you may want to wait till the Intel Price Cut, i think its happening sometime this month (Backup on this guys ??)
Thanks, have good fun with the build