Performance Difference

~15% boost in performance, if you throw in a second 4870 in crossfire you would get about a 70-80% boost.

Not a ton, except for some DX11 games as you might get better multi threading. For that price point, you should look at a GTX460. Otherwise go a level up to 5850.

EDIT: Or CF like hunter said.

i was thinking of crossfiring 2 5830's im not a hardcore gamer i would like dx 11 and would 2 4870s run on a 650w corsair power supply?

Yes 2 4870s should run on that, it's a good PSU (650TX I'm assuming).

2 5830s would be a good set up. However if going for 2 cards, then I really, really recommend the GTX460 1GB model ($230). In all the reviews I've read, SLI with them can at times match CF 5870s. How? I don't know, but Nvidia pulled a diamond out of their ass with this one.