Well, If you get essentially the rig I'm running right now, then your looking at about "974.61 USD"
I'm able to run everything out there so far at reasonable graphics (crysis at medium, highest texture, low post).
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131568R
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102037R
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103584
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227210
http://www.newegg.com/Product/Product.aspx?Item=N82E16817104015
http://www.newegg.com/Product/Product.aspx?Item=N82E16811133021
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136178
http://www.newegg.com/Product/Product.aspx?Item=N82E16835185038
http://www.newegg.com/Product/Product.aspx?Item=N82E16811999344
The opteron will OC to about 3.2 (its dualcore) with the right cooling (vapochill or peltier), I'm running mine at 2.6 on air (Though I have a 170, still dualcore).
X1900xtx can run AA and HDR at the same time.
I would suggest getting the armor case, it has massive amounts of room for modding, and a backboard so you can run your wires behind your mobo. The HDs are also placed by the PSU so that you get fresh cold air into your rig through the front. Infact, I'm going to mod my case soon so I can fit a Vornado fan on the front of the case (small one) and make it look like a jet intake.
The Heatsink (and fan) are essentially for OC or keeping your temperatures nice and low. Although you could get a better Heatsink (like a Thermalright ultra 120 extreme), I use a Ninja and that fan combo and I get temps of 29C idle and 34C max load. If you go with a better Heatsink, use the suggested fan, It moves the most air out of all the 120mm's.
If you want a bleeding-edge computer, off the top of my head, It should run you anywhere from 1600-2000 USD. Though there are some things you can change in my suggested rig and be around the 1200 area. (Mid-post edit, wow 800 pounds is alot of money USD, I didnt realize the exchange rate is so much).
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115017
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227178
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131073
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148278
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148151
http://www.newegg.com/Product/Product.aspx?Item=N82E16817104041
That quad core I've hear OC pretty well (up to 3.0 with proper cooling, and sometimes not proper cooling). Multiple cores are starting to get supported alot more now (crysis, alan wake), so you may want to invest in one over a dual core. Of coarse youll need a good mobo to go with it. Since I have an Asus, im a little biased, I think the BIOS offers a whole lot, and it seems very stable. Mobo discussions are very hard, since its so far ranging.
As for the HD's, 1TB gives you a whole lot of room for all that porn (and maybe some games as well). Running the two 500gb's in a Raid configuration would make it faster, but it might just be easier for you to just get 1TB'er.
As for the PSU, 1000W might not be needed (700W may be enough), but more power than you could possibly need never hurts.
Your around 1310.91 with all that new stuff, which just leaves you with a graphics card choice, which will run you between around 200 to 600 USD, which puts you under to above your preferred budget, depending on your choice. Also, be sure to look at derivitaves of the products (may be cheaper), like OEM's, and the Armor case without the 25cm side fan and such. Really, just do a little research, in order to make the best decision with the information at hand.
http://www.ocforums.com/ This forum is really good for anything computer related. They critique any proposed build to make it fit you better, or just make it better in general.
Edit-- Ack, forgot DVD drives. They can be really cheap (price wise), but check into brands a little before you decide, certain brands have certain reps. Make sure you get what you need though: single-layber vs double-layer vs blu-ray vs HD-dvd.