The Dell is good, but as you rightly found out, except in one important department - the relatively poor graphics card.
As a gamer, & for a budget of £700 you should be looking at an ATI 5xxx series card. I'll get back to you on a few possible systems.
Dell is probably one to avoid as their gaming system ranges - XPS, & more recently Alienware - can be a bit on the over-priced side. G