Why not get faster ram and spend your money on the cpu get an i5. Instead of 2 gtx660s.
Fast ram would be nice, but the 6300 offers just as much power as the 3570k. There like a very slight difference in frame rates. The SLI GPUs would definitely help with most games. My recommendation for that however is to get a gtx 680, since two 660ti's are about as powerful as a 680 and you don't have to worry about scaling issues
The GTX 680 is also another GPU to choose from,along side the Radeon HD 7970. I figured in a lot of games people play, in a good selection of games 2 GTX 660 TI's could beat a single GTX 680 and Radeon HD 7970
1 card is always better in my experience, Ive used SLI and yes in benchmarks that utilise its capablilities fully the SLI setups usually win, but for gaming and applications, It seems that one single powerful gpu will out perform the SLI setup in generall as some games may have issues with SLI or just simply do not utilise it completely. Also I would probably switch for an Intel CPU also imo better performance overall.
On a side note If I had 1000 blips right now this is the build I would go for.
http://pcpartpicker.com/uk/parts/partlist/