The downside with having a overspecced PSU is many.
its a lot of money down the drain
It will be loud
It will be hot
You will get a large powerbill becouse its large and probably not very efficient if you only draw half power from it.
A PSU will only generate what is needed. If your PC requires 400W a 800W PSU will generate only 400W. This is no sweat for a 800W PSU so it won't be loud and it won't be hot.
The closer to the PSU's maximum output you get, the less efficient it gets. A 800W PSU will be more power efficient than a 450W PSU at generating 400W, assuming they are of same quality.
With this in mind, for the OP's system, I would get at least a 550W 80+ Gold-rated PSU for the current system and at least a 850W 80+ Gold for the SLI-build. The 750W PSU will be able to generate the power needed, but will be inefficient.
I had fun with 2012, until I started playing online. The game was full of trolls that would just stop when you were about to finish them. I left it going one time, for three fucking hours and that fucker still couldn't handle defeat. I was hoping for a better game this year, too bad.