As citadel mentions, for the 8800GT the CPU may be a little lacking / may cause a bottleneck - basically the graphics card won't perform to it's full potential. You'll probably need a better PSU too for ~£40-50 extra*. What PSU do you presently have - make, model & wattage?
If budget is a concern, go for a 512Mb HD3850 from Scan for £102 delivered (see my post in this thread click here). Again, the PSU may be a concern. G
* eBuyer do an adequate case+PSU for £49 delivered click here. Free delivery applies if your order is over £50 or over.