Hi,
Ideally you want to be looking at either an ATI HD4850 or an Nvidia 9800GTX+.
For a HD4850, best ones to get are either the HIS HD4850 ICEQ4
or
PowerColor PCS, as these offer better coolers and are alot quieter as well.
Shop around there are some good prices if you compare E-tailors.
Performance in decending order and rough prices & rough performance taking the GTX280 AS 100%
GTX280 £300+ - 100%
HD4870 1Gb £200+ - 90%
GTX260 £200+ - 88%
HD4870 512Mb £180+ - 85%
9800GTX+ £135+ - 70%
HD4850 £120+ - 65%
9800GT £90+ - 50%
These are only rough but should give you an idea, 4850 & 9800GTX+ are best bang for buck.
Cheap 4850
http://www.novatech.co.uk/novatech/s....html?NOV-4850
Cheap 9800GTX+
http://www.novatech.co.uk/novatech/s...tml?NOV-98GTXP
Decent/Cheap 4850
http://www.aria.co.uk/Products/Compo...roductId=32374
If you can stretch i would get this one
http://www.overclockers.co.uk/showpr...)%20-%20Retail
You should get about £20 for your 8400GS on ebay.
Make sure you have at least 550W Power Supply.