Both are good choices, both will overclock well. The phenom has a unlocked multiplier so overclock is a little bit easier.
Either way you, go nobody can say you made the wrong decision.
In my opinion, if you have the money to spend on the i5, then do. But do not take money away from the other system components in order to do so. Because you don't need anything more than a phenom II 955 or similar.
On a side note, a 1000W PSU is way overkill. You don't need more than good 500W.
I'm planning on running Crysis warhead on high.
At what resolution? And what framerate is acceptable to you? I can do that a 1280x720 on a radeon 3870 with 30FPS. But certainly not at 1920x1080.
For that, make sure you get at least 4GB of memory. Even with 4GB, Warhead crashes on me if I try to increase the AA past 4x. When I had 2GB of ram, it crashed when I increased the settings past medium-high.