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.