SLi only gives a 30-70% performance increase when it should give a 100% performance increase as you are paying for 2 instead of one.
A lot of people here will tell you that SLi is a waste of money however if you think about it, it's just like buying anything else. An AMD Athlon X2 4800+ is twice the price of an AMD Athlon X2 4200+ but it gives no where near a 100% performance increase but still people here will recommend it.
Whether or not SLi is worth it is up to you and how much money you are willing to spend on it.
Intel Core 2 Duo E6400 @2.4Ghz
Connect 3d X1800XT 256mb
G.Skill 2GB DDR2 667Mhz
Belinea 19\" Widescreen LCD
Antec Super Lanboy
Seasonic ATX v2 430W