Yeah 700, is a nice budget for video cards. I realy think you should go with the SLI 7800's just make sure you have a good enough psu, might have to get another one most sli configs need a lot of power
i wouldnt sli 2 7800gts because im not sure that would be as good as an ati x1900xt, i would go with the 1900xt since its cheaper and that card kicks ass...oh and make sure u have a pci-e slot before u buy
i know, if i were you i'd buy a 6800gs, and a new processor. what are the rest of your specs? you'll need some good cooling to sli 2 7800's together, and not to mention a good power unit.