it would not make any or very-very little difference. Negligible in my opinion.
It's said that a GTX 512 is about 5-to-10% faster (yet it cost an arm and a leg) than the regular 256.
I think Â£180 = $250-$275?
For that cash, I'd go for a 7800 GT instead. That would really make a difference.
Personally, I'd say it's not worth it, a regular 6800 GS would do great, just overclock it. A 6800 GS overclock to 1200 mem, it's pushing 38.4GB/s in mem bandwidth, and the GPU at 500mhz makes a bit of an improvement in the rest.