well i think sgt is probably the man to help but for 700 gbp arond 1000 bucks I got
i7 2600k
8gb ram
2tb hdd
blu ray
gt 545 3gb
the system I got is pretty decent spec, the only thing that lets it down is the graphics card, 3gb sounds great but its not what counts. Going to just get 550 ti card and swap it as my power supply should handle it.
Try websites like these, although i do not know the american sites
http://www.overclockers.com/ you might find good website to buy your own spec or premade bundle here
These are just some quick US searches I done, take your time and shop about,
I think think you want to focus on graphics and processor
Maybe a 550 ti or gfx 560 (sensible decent gaming cards)
As for cpu then the latest i5's are more than enough
Ram the higher the better but I would aim for 6 or more
http://www.cyberpowerpc.com/http://ironsidecomputers.com./Here try this, I prefer nvidia so you can just change the gpu if you prefer one or other
http://ironsidecomputers.com/index.php?main_page=product_info&cPath=3_13&products_id=21