Considering that SLi does have CPU overhead to it, and you are using an older and slower single core CPU, I would say it is. If only slightly, and depending on the game you are playing. If you upgraded sticking to your current socket (939?) and going to say a reasonably priced 4800 X2 I think you would see a modest increase in minimum framerates and most likely a small jump in average fps.
The socket 939 4800's arent to bad, the socket AM2 ones are a bit more. Look on ebay and such, I'm sure you can find one in the low 200 range. For the power you get thats an awesome value. 2.4ghz per core with 1mb of cache per core, you can easily overclock them to 2.6 - 2.8 ghz per core if you are so inclined.
Its a great setup, and will get even better with a newer dual core CPU.
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