No it doesnt require a 750W psu. Something around 450W is fine.
It really depends how you understand bottlenecking and which games you play.
You will
gain extra performance but it really depends. Some games such as
Sleeping Dogs are absolutely taking nothing from the CPU and are obviously GPU bound games where you will see an improvement over 6870's. But if we across games like WoW you won't see any gain simply because it's a cpu intensive game and even the latest blingbling cpu's do bottleneck 280x's in 25m raids.
It's not a bottleneck in general, you'll have acceptable gpu loads but you'll see there's more potential left. You can test it by yourself out. I'll give an example: just join a (empty) server in BF3 and monitor your gpu loads with something like msi afterburner and make sure to disable vsync. If your gpu loads are nowhere close to 80-90% then you won't gain any performance in BF3 with a 280x upgrade.