Games running choppy or top of screen only in SLI mode

I just bought another GTX 550 Ti video card in hopes of running both cards in SLI for better gaming FPS. When I enable SLI. Game play is very choppy or I only see the top half of the game. Also, when I disable SLI my games run fine. I just didn't know if anyone knew why I'm having problems in SLI mode ?
This is what I have for a PC:
My Computer is as of 11/3/11
Motherboard is: ASUS, M2N32-SLI, Deluxe
AMD Socket, AM2+/AM2
NVIDIA nforce 590 SLI
2xPCI-E x16 Slot,Nvidia SLI Support, At Fullx16x16 Speed
Dual Channel DDR2 800/667/533 Memory/Ram
Bios updated 5/3/11 to 5002
CPU/Prossesor is: AMD Athlon 64x2 Dual Core Processor 6400+
P/S is a: Ultra Xfinity Dual Rail Technology 600 Watt. ATX, SLI ready
Ram/Memory is:
Total Memory is : 3070MB
4 sticks total
2 sticks are: 1024MB,667mhz(speed) 64bit
2 sticks are: 512MB,667mhz(speed) 64bit
2 Hard Drives:
Maxtor S TM3200820AS SCSI
WDC WD32 00AAKS-00B3A
DVD/CD Drives:
TSSTcorp CD/DVDW SH-S182D
ASUS SDRW-08D1S-U USB
Grafix Card's 2) One Evga Geforce GTX 550Ti and one PNY Geforce GTX 550Ti as of right now. They are not running in SLI mode. SLI Disabled.
Sound Blaster Sound card
Monitor is: ASUS 24" HD Running at 1920x1080px32
OS is: Windows XP

I also just ordered a new CPU for my motherboard. AMD Phenom IIx4 955 Quad Core Black edition.

I hoping the problem with running SLI is my CPU getting bottlenecked from the two GTX 550 Ti video cards in SLI mode. I was running two GT 9800 video cards before in SLI with no problems.
Also, when I disable SLI my games run fine. I just didn't know if anyone knew why I'm having problems in SLI mode ?