id defo say thats the right choice. the 6750 will easily reach the same speeds as the 6850 if you want to go that way. In the days of the E6X00 series, the E6800 was multiplier unlocked, therefore the hardcore gamer and overclockers choice, however the E6850 is not multiplier unlocked, therefore i see no real advantage.
So currently alexnifty and I would recommend changing the following (just to provide an overview, lol)
X1900 OUT, 8800GTS 320mb IN
Asus P5K OUT, Asus P5N32-E SLI IN
Intel C2D E6850 OUT, Intel C2D E6750 IN
Original cost of those three items = £335.04 (ex) vat
New cost of replacement items = £370.00 (ex) vat
Its not an awful (£34.96 ex vat) lot more for just under twice as much graphics processing power!!! (see http://www23.tomshardware.com/graphi...=778&chart=275 )