What makes you think your performance is not what it should be?
Are your getting too low frame rates in game? What games? What FPS are you seeing?
SLI only offers a small performance boost in some games , maybe 5-10%. Other games will see as high as 50%.
Other items such as too little RAM or running RAM that is not set up for dual channel can slow performance.
We need the exact system details:
-Socket 754 or 939 processor, (socket 754 memory controller is brain dead)
-AMD 64 or AMD 32 BIT processor, (probably AMD 64)
-how much total ram, (need at least 1Gb in 2 similar sticks)
-is the RAM in 2 sticks of the same size, rating,
-MB make and model
A program called CPUz can read your system and provide the information if you don't know. Free download at majorgeeks.com