Whether it's worth the expense is very subjective. What is your E8400 clocked at? How much RAM? What games do you play/want to play and at what resolutions? That may narrow down how much gpu muscle you really need to get very playable framerates.
The 6950 ($280ish right now) is a bit higher end than the 6850 ($180ish) / 6870 ($210ish). Either of those would be excellent options. There are also some great deals on 5850's right now as well ($180ish), if you don't mind getting a card that has been out for a little while. If you want to consider green team (they're good too!) the 460 and 470 are both great cards with pretty good deals out there. Just keep in mind that you will need to get a pretty solid OC to get the most out of these cards- a lot of games won't suffer too badly from having only dual core but stock clocks will put a dent in your performance.