I just read this roundup:
http://www.firingsquad.com/hardwar [...] efault.asp
because I'm looking to upgrade my card to an 8800gts 320mb.

which one should I get? I'm looking to spend around £200.
for the overclocked versions of these cards, couldn't I buy a standard card and overclock it myself?
are there any aftermarket coolers available for these cards?

thanks.

getting factory overclocked version of the card will guarantee you better result than the default ones in most cases, because they handpick "good overclock potential" cards and make us pay more for them.
I think overclocked version will cover you some overclocking warranty, meaning should something screwed up, they can get covered.
Personally I would get evga or xfx for their exception lifetime warranties and customer services.

Another suggestion would be to give it a little wait, the last time I checked, 8600gts would come out around April 17, it is about the same performance of 7950gt for around $200 US .