It will depend on whether a game is more CPU or GPU dependant, you'll need to look as specific benchmarks for games you're interested in and make a decision from there.
Remember than anything over 60fps is not visible - on a 60Hz monitor anyway.
40fps is considered adequate by most reviewers but I certainly prefer the fluidity that 60fps provides.
I doubt that the Phenom II would bottleneck a 5850 and you still have plenty of overclocking headroom to play with anyway if it does.
The i5 is certainly a more powerful CPU but the P55 chipset provides fewer PCI-x lanes than the AMD 790 chipset which could be an issue if you're looking to run multiple graphics cards.
A game that i have in mind is battlefield bad company 2. I have seen that 4 threaded cpu's add about 10 fps over a 2 threaded cpu in that game.