Thanks guys for the replies, is the 4870 quite an upgrade from an 8800gtx?
Yes it is an improvement if you are playing at high resolutions.
Pleas note that the link comparison provided by Oakley707 is a year old and is way before Cat 9.x drivers were out. Performance and stability of HD4870 through drivers has increased since than. Plus these benchies were done with AA at 0. Once you enable AA at high resolutions 8800GTX gets a really big hit in Frames per Sec. making it unusable for some games....