Need help to find bottleneck and solve problems in my new homebuild!

I've just finished my homebuilt system which consists of the following parts:

CPU: AMD Athlon II x4 620
Mobo: Gigabyte GA-MA790X
RAM: 2x2Gb A-data DDR2 @ 800MHz
PSU: Coolermaster @ 750W
HDD: Western Digital Blue Caviar 500GB
GFX: 2 pieces of ATI HD4870 512 Mb running in CrossfireX
OS: Windows 7 Ultimate 64-bit

Problem:

I suspect a severe bottleneck. When I run 3dmark 06 with standard settings on my system, I get around 14000 points, which is well below what you could espect. When I turn CrossfireX off, I get around 11700 points. So there's only a 2000 points gain. Also, I bought the GFX from a friend who had a similar system with a Intel Qxxxx quadcore CPU and got around 20 000 points. Also he could run the newest games, for example battlefield bad company 2 on highest settings in high resolution. I can barely run BFBC2 on medium.

The most likely bottleneck I find is the CPU. Though there's a problem with that theory. When I check benchmarks where they have tried the same system with different CPU:s it only differs on maybe a 1000 - 2000 points in 3dmark 06 between the Athlon II x4 620 and the Phenom II x4:s and some of the intel i7:s. Correct me if I'm wrong, but that's what I could interpret from some of the benchmarks. But still, the CPU is the most probable bottlenek. Or?

I would be deeply thankful for all the help I could get. I just upgraded my system and kept the CD/DVD, HDD and processor but bought the rest. Which costed some money of course. And it was in purpose to be able to play new games. Which I should be able to. So please: is there a possible solution to the problem? May I gain a lot of performance by changing the CPU?

Sincerely
Robin