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(oops). Yeah or the above but I'd rather get the 8800GT as an upgrade. Especially if you like AA. The 4850 would be the best option it looks like but a bit dearer than what you've specified.It would have to be a 3850 then - the 4850 might be too much of a stretch if you are strictly under 100 quid.
Maybe this
3870 should be around for sub £100 new?
Also, you could wait for 4850 to possibly dip to £100, the X1950XTX is still a decent card depending on how picky you are/ what res you game at

whats the 4850 like compared to the 8800gt?

Going by this :
http://www.hardwarezone.com/articles/view.php?id=2351&cid=3&pg=6
A 1950XTX gets 33fps in Fear @ 1600x1200/ 4xAA / 8xAF with a 2.66GHz C2D
An HD 3870 gets 35fps in Fear at the same res / AA / AF / cpu :
http://www.hardwarezone.com/articles/view.php?id=2501&cid=3&pg=4
If that's reasonably accurate, I wouldn't consider the 3870 as it's hardly going to give you much more than what you have. Has to be the 4850 or a clocked 8800GT at least.
Going by this :
http://www.hardwarezone.com/articles/view.php?id=2351&cid=3&pg=6
A 1950XTX gets 33fps in Fear @ 1600x1200/ 4xAA / 8xAF with a 2.66GHz C2D
An HD 3870 gets 35fps in Fear at the same res / AA / AF / cpu :
http://www.hardwarezone.com/articles/view.php?id=2501&cid=3&pg=4
If that's reasonably accurate, I wouldn't consider the 3870 as it's hardly going to give you much more than what you have. Has to be the 4850 or a clocked 8800GT at least.