I've visited the SLI faq and read my board manual to make sure it can support it, which it said it can. I have been looking into an update to my graphics card, I currently have the GTS450. If I get a new graphics card, I can only afford around $100 for one. But I can easily find a used GTS450 for around $50. Would WoW benefit from doing SLI with the 450? As far as FPS goes? I can almost play on Ultra as it is, so I only need a small boost. Which means I DON'T need a $200 graphics card. So my options are SLI the 450, which is preferential if it will help me. Or get a 560TI.
If you're happy with what you have, then stick with what you have. Save up for a single card for now.
What's the rest of the system you have?
SLI and CF on such low-end cards tends to not work well except with Radeon 77xx cards and maybe also with the GTX 650. Higher micro-stutter is common and it can be very annoying and detrimental to the experience.