The E4300 and E6300 can overclock equally far, but it's not worth it for the mere $10 you save on getting the former.
I'll tell you the same about SLI that I've always done. Don't do it now, don't do it in the future. For a budget gamer, it's too expensive and the benefits are too little. Most of the time you just get around 10% performance increase, against a 100% price increase, heat increase and energy consumption increase.
Yes, SLI is popular among some people but to people like me they just look unwise instead of cool and 'zomg it's teh sli!'
And in that light, I recommend you change to a non-SLI board (actually it's the chipset that makes me recommend a p965 board). For that price you can get a Gigabyte 965p-ds3 which is a better overclocker and a more stable motherboard with better support.
Also I believe this X1950Pro
for $154 after mail-in rebate will serve you a little better than the 7900GS you've picked. Which is another excellent card, but in my opinion 7950GT > X1950Pro > 7900GT > 7900GS. Don't get this card without changing the mobo because you can't run this in Crossfire on a Nvidia chipset. But you can run HDR and AA at the same time when playing games.