There is some 7800 cards with 512mb RAM going around these days for under 100 quid.
I think Gigabyte are the ones doing them.
Or you could get a couple of older series 7xxx cards or Xxxx ATI and SLI them.
EDIT: Sorry 7600 not 7800
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http://www.dabs.com/productview.aspx...avigationKey=0
Cheapest I could find for that card.