Here's a review of the hD5770, the benchmarks cover most of the cards in your price range that would be a useful upgrade:http://www.techpowerup.com/reviews/Powercolor/HD_5770_PCS_Plus_Plus/18.html
Also have a read here:http://www.tomshardware.co.uk/graphics-card-radeon,review-31911.html
Your card is nine down on the left hand side of the chart at the end BTW. Anything three tiers or more over the 8800 will be a good upgrade.
You will not buy a card that your CPU cannot keep up with for £100, you'll be fine.
The GTS250 is a good card for your needs.
Also look at : Overclockers, Scan, Misco, Microdirect, Dabs and CCL to widen your search.
Edit: Nice find by Jyjjy