You can check out the benchmarks for that card in SLI, you can get up to 80% performance increase. If you do go SLI you should re-install the latest drivers after installing the second card.
My GTX460 overclocks like crazy, I haven't tested the highest clocks yet but I can get into the high 800's so far. I don't have a need for SLI yet at the resolutions I use (1600x1200).
I used to be against SLI due to the fact that the performance doesn't double, but if you need high performance on the cheap SLI is the way to go. Sometimes its cheaper to sell the old card when there are deals on new generation cards but right now the GTX560 is only %10-15 faster than a gtx460 so thats not worth it at all.