They're costing £50 - £60.
Just found an XFX 8600GT Fatal1ty 256DDR3 running @ 620Mhz which it claims is the fastest 8600GT. That better than higher RAM?
Not sure about UK prices, but at the 1.8x exchange rate, that would be a horrible deal by US prices. You can get a far better performing 9600gso/8800gs for less than that. Search around for better deals.