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I'd say 4890 if you're desperate to upgrade now with the cash you have, however I feel for more longevity you might want to save a little longer and grab yourself a 5850![]()
The only upgrade for a 8800GTX is a 4870x2(4872) to gain any noticeable increase in gaming power, or a 5850. The 5770 is on a par or slightly slower than a 4870 and your not going to notice much of a difference.
The 8800GTX was Nvidias best GPU before the GTX260+ entered stage, thats why they re-badged it and tweaked it to creat the 9800 family, so keep your 8800GTX and give it a bit of am overclock, perhaps even buy an after market cooler for it so you can crank it some more, and meanwhile save up another £60 for a 5850 or 4872