AM3+ compatible with AM3 boards?

Hello everyone,

I'm in the process of helping a friend fix his PC. Since guild wars has come out he was looking to upgrade, he recently upgraded his video card from a Radeon HD series 5000 series to a 7870. He also upgraded his PSU from a safe power 500W (it was a pre-built computer from cyber power PC I think) to a Corsair GS700W. He's currently running an AMD Phenom II x4 925 Processor, ~2.8GHz. He also upgraded some RAM but ended up getting it DOA.

Now more on point to the subject, overall he says his computer seems to run slower, but as someone who has played on the inside of computers for a while I'm not sure what would cause the problem. Right now I have 4 x2 GB Corsair Dominator RAM in it now, figuring that might help, but it really doesn't. The only thing I can think of is the CPU is lacking now, it's only a 2.8GHz, but still a true quad core. Would upgrading his CPU help overall, and if so would you recommend running an AM3+ CPU on an AM3 board. I have read that some AM3+ CPU's are compatible with AM3 boards, just that it's not officially supported by AMD.

Lastly on the RAM I recommended he pick up either Corsair vengeance or G-Skill Ripjaw series. Is there any difference in running 4x4gb compared to 2x8gb sticks. I know the mobo can handle 16GB's, but would there be any noticeable difference in running 4 sticks compared to 2.

Thank you for the reply.