Unless you're getting a really good deal on them, or you've got full-cover waterblocks that you don't want to throw away, those video cards don't really have much advantage over the newer 8800 GT/8800 GTS 512 cards, in price or performance.
Might want to consider a 2x2GB configuration for your RAM. Most boards, even the ones that run 4x1GB without trouble, tend to be more stable with the 2x2GB setup, especially when overclocking.
Technically, your PSU could probably run an SLI setup, but it'd be pretty borderline, and dependent on that PSU actually delivering what it promises. And for what it's worth, I can't find the 600W version of OCZ's GameXstream on Nvidia's list of SLI-certified PSU's - Just their 700 and up. OCZ says it's SLI certified, but having the extra PCI-E connectors is just the first step. Might want to look at a 700-750W PSU for peace of mind.
i7 2600K | ASUS Maximus IV GENE-Z | 580GTX | Corsair DDR3-2133