You can get 2 gigs of DDR3 1066 for like $102 on newegg, but honestly, the 790i isn't even worth it at this point, with a 780i and 3 9800GTX you can achieve the same performance and even better in some situations as a quad SLI setup with 9800 GX2.
Also the price of 3 9800GTX is cheaper than two GX2