7600gs 512mb in sli mode! performance question

Ok so my current setup:

Amd 64 am2 x2 5000+ BE

Corsair TwinX DDR2-800 4-4-4-2, 2 x 1024mb

MSI K9VGM-V Motherboard (its bios dosnt allow overclock which is **** for my BE 5000+ )

PNY Verto 7600gs 512mb


Ok so my question is, ebay seems to have 7600gs's cheap like probably can snag another one for $30, my plan is to sell my current mobo to my friend since he is buying my 3800+ x2, and im thinking of buying the MSI K9N sli mobo. Would it be worth it for the extra $30 to buy another 7600gs to throw in there and run sli? only game ill really be playing is Warhammer Online, i played beta and full settings it ran great, but my logic here is.. if i can see a good performance gain for $30 i might as well use the technology that i have avaible to me no?

also in order to run SLI do the cards have to be identical in every sense of the word?