Is it possible my CPU is too slow for my RAM?

I recently "upgraded" my self-build with a new mobo, APU, and RAM.

My old CPU was an AMD Athlon 64 X2 5400+ Black Edition - 2.80GHz
My old GPU was (2) XFX GeForce 9600 GSO Video Card - 768MB DDR2
My new APU is an AMD A8-3870K Unlocked Llano 3.0GHz Quad-Core APU (CPU + GPU) w/ Radeon HD 6550D

My old RAM was Corsair TWINX 8GB (4 x 2048MB) DDR2 800MHz (PC2 6400)
My new RAM is G.SKILL Sniper Series 8GB (2 x 4GB) DDR3 1866 (PC3 14900)

My brother is now using my old components. We play Diablo 3 together, and I lag down to about 6 FPS at times when there are a lot of moving objects (barrels exploding, spell spamming, etc.) while he does not.

I thought maybe the Radeon 6550 that's built into the new APU was weaker than the old GeForce 9600, so I took one of the two old GeForce cards away from my brother and plugged it into my new configuration. My lag problem persists. His gameplay actually improved.

So now I'm wondering if...

1. I have a defective APU (I am past the 30-day return period but I think I can still buy an extended warranty through Newegg)
2. The RAM is too fast for my APU's processing power (would overclocking be a good strategy?)
3. I don't have enough power for the new components (although I'm pretty sure they're more energy efficient)
4. I have a bad installation of Windows 7 (I've gotten 2 blue screens in the past month and my computer has locked up a few times while playing Diablo)

Any educated wisdom would be a delight. Thanks, Tom's Hardware!

EDIT: simplified specs, noted fps

EDIT 2: It takes about 15 minutes to boot and sometimes won't boot at all. Often I get a disk read error before the "Starting Windows" logo even appears. IF I get into the Windows OS, I get the blue screen of death upon trying to open anything. IF I successfully boot to Windows Setup Disc and select the Repair option, it doesn't recognize my Windows partition at all.