If you are going to count fps and stuff like that then you will however most people play games with budget cards (mainstream) that's the most profitable sector of the market for card makers!
click here see this one from novatech choose the 3800 and it's £549 inc VAT for the system and OS. That leaves you to buy a monitor with your £150 left over from you £700 budget.
AMD Athlon X2 3800 Dual Core 64 Bit Processor
1024MB PC3200 400Mhz DDR Ram
250GB Serial ATA II HardDrive
nVidia 7600GS 256MB PCI-E Graphics Card
16x Dual Layer +R/-R/RW DVD burner
7.1 8-Channel Capable Sound
1 x Gigabit LAN
Logitech Wireless Optical Keyboard & Mouse