is SLI Really worth it?

im considering buying a new system soon, i like to get the best for my buck cuz im not a very wealthy person.

anyway i wanna know your opinions on SLI, heres mine.
sli clearly cant be more then 2x better then 1 card, so its probly around 190% of what one card would be.
it also takes some cpu power as it has to split up the commands, but not much.
for example heres a price comparison.
it costs 20$ more for your motherboard to support sli.
how would 2 6600LEs compare to a single card.
6600le costs 95$ according to newegg.

for 190 we could also get
a basic 6800
or a x800

with 2 6600le's we have
8 pixel pipes
512 ddr400 memory
pcie 16x split between the 2
2x the heat...

with
an x800
we have 12 pixel pipes
256 ddr3
pcie 16x on its own.

but how would they compare in performance?
anyone use sli?