Crossfiring low-end cards might equal a medium-end card. Maybe. But low-end crossfire is particularly vulnerable to micro-stuttering. Just save up for a decent card.
If performance on your current card bothers you, you can get a 6870 that will be a major improvement over what you have (probably better than the 6750s in crossfire even if you get the crossfire configured & working perfectly).
$175-$30 rebate - $145 total
If rebates bother you, you can get a cheap 6870 from newegg:
$155-$10 rebate + $8 shipping + free Dirt3
So you can significantly upgrade for about $150. If your monitor is 1680x1050 or less, the 6870 should blow away even the harder games to run. At 1920x1080 it'll still be great for most games and run the harder games smoothly on 'high' (though not smoothly on 'ultra'. If your resolution is 1920x1080 it might be nice (if in reach) to save for a 7850, but the 6870 a fantastic performer for the price.
The 6750 is very nearly the same card as the 5750. You can see the 5750 compared with the 6870 here: