SLI vs dual cards

Sorry if this has been asked a billion times before, but i couldn't find a thread that answered my question after a search so here goes:

I have dual Quadro FX 1500's on a non SLI board. I can upgrade to a single Quadro FX 3500 if i choose, but i don't know how to figure out if that actually constitutes an upgrade. yes, i know i could run all sorts of benchmarks with the 2 sets of cards, but i thought maybe i could just tap the wealth of knowledge on tom's forum.

i checked the vid card charts and the fx 3500 always edges the fx 1500 by a pretty slight margin. the question i have is how do i determine whether or not the two fx 1500s i have would be outperformed by the fx 3500? is there that much gain from running dual cards? or is the gain only from running them in SLI mode?

thanks in advance for any help!

ahh. ok. so the only thing that i'm getting from the second fx 1500 is the ability to hook up 2 more monitors (which i don't do anyway)?

also it doesn't matter what applications i use right? its not like XSI or Max or Maya are written to somehow spool out work to two cards if it detects them or something?