Ok well get a new psu with your upgrade. And just buy a x1800xt and overclock the hell out of it. If you are really set on going nvidia pick up a gtx now, and if you need it later you can get another when prices drop. (gt's and gtx in SLI require a VERY overclocked cpu to not bottleneck , especially if the cards are overclocked)
If you plan on using your LCD, i know its resolution only goes to 1280x1024 . Running sli setups is really for people that use 1600x1200. A single gtx or xt will be more than enough to run games at 1280.