Keep your 6600 for a little while. The 8800GTX and GTS are due soon and I'm sure that the price of the 7-series cards will fall soon thereafter.
My 256MB 7800GTX still packs a real punch. If you wait a while, I reckon you will be able to pick up a 512MB 7950GT for under £150.
My 256MB 7800GTX still packs a real punch. If you wait a while, I reckon you will be able to pick up a 512MB 7950GT for under £150.