To be honest mate I don't think they're massively different so it's up to you really. The 9800 I posted is only £1 more than the 8800 and comes overclocked, but it has a different type of RAM on it.
It pretty much comes down to how much you want to spend. You can get this higher specced 9800 for £120 or you could get the lower specced 9800 or 8800 for around £100.
EVGA 9800 GT 1GB PCI-E Dual DVI Video Graphics Card - 01G-P3-N972-TR - www.microdirect.co.uk