Still very mediocre upgrade, a downgrade in some cases like mass effect and most prolly other UE3 engined games. I wouldn't waste my time and money personally unless I'd get at least a 50% boost. Really, save up for a HD4870, once you have it you won't regret it.
No point buying a G92 based chip now or a 4850 which is meant to compete with it. Do you really want to spend 100 quid on an upgrade for a -10 to +20 % performance difference depending on game ?
Sell the 8800GTS for 65 quid, add 100 quid, and buy a HD4870.
http://www.overclockers.co.uk/showproduct.php?prodid=GX-091-PC
that is a good upgrade.
A 4850 or 9800GTX is not a good upgrade, it's a waste of time.