Only "cheap" sli system worth even bothering with would be 754 Epox SLI board, 3000 athlon 2.0ghz, dual evga actively cooled overclocked 7600gs's.
!! I have editd this post because I was a total D@#K to shabodah for no good reason !!
I feel there is no such thing as a viable budget SLi option
The 7900GT is going for ~ $300
The 7600GT is going for ~ $200
two 7600GT's is cost ~ $400
you may get the speed of 1 7900GT if you're extremely lucky, but it will cost you $100 more, and a jump in your MWh