SLI'd EVGA nvidia gtx 260's (216 superclocked model)
OCZGXS 700 watt PSU (soon to be replaced with Antec 1kw)
Are you making any other changes so that you'd need a 1KW PSU?
GTX 460 actually uses less power than the GTX 260s. Even the factory OC'd GTX 460 models are only a few watts more in some cases than the stock 260s. Your 'superclocked' GTX 260s probably use more power than the 460 replacement's would use. GTX 260 vs GTX 460 power reqmnts
With no bottleneck? They're not 'magic' video cards so don't expect miracles - they're just a bit more powerful video cards.
You can look over the gaming benchmarks in the rest of that article linked above. Here are some 460 SLI benchmarks
What screen resolution do you use?