Yeah, it's already purchased, @skora, you said it would be better, but would it still be okay. Let me put it this way, I have an evga 9800gt superclocked, would I notice any difference... lets say in Crysis by switching to 2 gts 250's?
I would expect the SLI setup to perform better but not much since the CPU will probably be cramping the GPU subsystem before you can stretch the legs of 2 GPUs.. BUT, I wouldn't expect 2x250 GTS to do that much better than a pair of 9800GTs. If the 250s are purchased also, I'd seriously consider selling them and the CPU, then using the money to get a new CPU and a second 9800GT. If you haven't bought them, spend the money on a second 9800GT and maybe the Phenom II x2 550.
What games are you playing? The time of buying a high clocked dual is as good as over. look at some of the latest CPU core scaling charts. Your best bet is to replace the CPU with something like the phenom 2 X3/X4s or one of the cheaper athlon 2 x3/x4s. What sort of budget are we looking at?