Why spend so much money on an SLI-rig when you're buying an old and and poor performing graphics card? If you insist on going with SLI then I would suggest that you buy either two 8600gt but better yet two 8800gts or better. But right now ATI gives you most bang for the buck. The Radeon HD 4850 is performing extremely well for a low cost. Buy either two of those or one/two HD4870 for a high-end Crossfire system, just don't forget to change the motherboard before ordering.
A 500W quality power supply is more than enough to run your suggested rig. If you want to have some extra power over for later I would recommend that you buy Corsair HX620.
If you plan on using XP/Vista with 32-bit then there's really no need for more than 2Gb of ram, since there is a limitation of 3Gb that the operating system can use. And with 3Gb you can't use dualchannel. Consider buying 2Gb DDR2-8500 ram instead, it costs a bit more but with todays low prices it's crazy not to buy it. Especially after you have just saved yourself a bunch of cash on the power supply.
I really don't see the point of the two 250Gb harddrives either, you get better performance with a single 750Gb and right now that size of hdd gives you best bang for the buck. Although it's not long before the 1Tb will take that throne.
Good luck man!
