Given build is an excellent one, I...third it??
Don't buy an XP license however, download the Windows 7 RC. Free OS for the next year and then you can buy a full copy of that. If you've read anything about it you'll know its going to be, or rather is, an excellent OS.
WD are great drives by all accounts. If you don't like them however there are always the Samsung F1 1TB drives to consider and far more.
The Intel Core i7 is the best processor series on the market right now BUT if you want to go down the AMD path then try the Phenom II x4 940 (and a suitable motherboard)
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more!
however this uses DDR2 memory hence you will also need to change the memory
Newegg.com - OCZ Fatal1ty Edition 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory Model OCZ2F10664GK - Desktop Memory
(Kingston HyperX is good, its not the greatest however and the above memory or the DDR3 memory will probably outperform it, but it is there if you do want it...
You'll also need to change the cooler but the rest of the rig will stay the same.
If you want an nVidia card, go for the GTX 275. Best price/performance card from them I'd say. Really top spec stuff. Otherwise get the HD 4890 or HD 4870
Hope this helps