I'm no tech-wizard, but from what I am hearing, I'd go with one better quality card instead of a lower SLI setup. Also, a 560GTX card that costs the same as a 460 is said to be as much as 33% faster. (That's just something I read, take it with a grain of salt.) I just installed a GTX 560 Ti card running at 900mhz for $250 and it works like a charm. Hell, if you have $500 to spend on a video card, get a GTX580. You'll be in dreamland with that card.
In any case, your setup is great, but the 460 is "yesterday's card", so I'm not sure I see the merit in spending the money to buy two of them.
Check out this link for some good stats.
http://www.videocardbenchmark.net/high_end_gpus.html
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