Need help on system upgrade

I have an intel E8400 chip clocked to 3.6ghz
Geforce 260gtx 1792mb
4 gig ddr 2 800 ram
Windows vista 64 ultimate


At the weekend I decided to upgrade my gfx card and bought a Radeon 5850.

However I cannot see much difference in games, the frame rates in ech game have increased by about only 5fps and my 3d mark 06 score has increased by 3000 points to around 18000 with the new card.

I was expecting much better performance - have I wasted my money in getting the new card given that it doesn't appear to be much better than my old 260gtx or is the CPU causing a bottleneck ?

If it is a CPU bottleneck what fo you recommend AMD or intel & what sort of chip ? I was looking at the i5 750.