There are 2 items I would change, unfortunately it would take you over your £800.
PSU - you should look towards 500W min (more if you want SLI), make sure it is a decent branded one with good 12V rails (Coolermaster, Seasonic, OCZ).
HDD - I would personally go for a SATA II drive with 16MB of cache (500GB drives are fairly cheap, or maybe 2 x 250GB drives in RAID 0).
CPU - do you really need a Core2Quad?, they are quite cheap but a Core2Duo would probably work out cheaper and most games are not yet Multi Core aware.
Other than that not too bad a system.