4gb to 8gb upgrade worse?

Hey i rescently just upgrated my memory from 4GB's of ram to 8 and my performance level went down on windows. I also noticed a bit of slugishness when i booted up Bioshock and played that to check the smoothness of the game. I read up on corsairs website and it mentioned that when 4gb to 8gb doesnt run well i need to slow my memory down and increase my northbridge voltage slightly. Im assuning i could do that through NVIDIA Control Panel.

Just to let you know what im working with
OS: Windows Vista 64
CPU: Intel(R) Core(TM)2 Quad CPU Q6700 @ 2.66GHz 2.67
Motherboard: XFX nFORCE 790i ULTRA (4 slots for DDR3 Memory; 8GB max; 1600MHz Max)
Memory: Corsair 4x2GB DDR3 1333MHz Dual-Path

When i 1st built the computer I had someone adjust the proper timing of the ram when it was at 4GB and im wondering WHat i need to chance to adjust my memory accordingly. Can anyone help me out with this one?

I use a Cool Masters 1000 SLI ready Power supply; and i have 2x9600 GeForce Cards Running in SLI. Im mainly concerned about mis-tweaking the timings and Voltages

Should I lower all the timings on the ram?

usually if all the ram slots are full, you put extra strain on the northbridge/memory controller. increasing the ram voltage by a bit, say 0.1v or 0.2v, helps alot. run memtest+ after you have increased the ram voltage to see if it's stable.