The phenom will do great for gaming, and will be able to match that 5870 pretty well.
However, if you are planning to crossfire in the future, the i7 might be a better choice.
I'd probably go with the phenom, though (or perhaps an i5) as it is almist as good in single GPU gaming setups as the i7, it is cheaper and I've realized that my plans for crossfire/upgrades rarely work out as planned and also because I prefer to get maximum performance right now instead of maybe getting it in the future.
The phenom build is also cheaper.
Both builds will do fine, though. The Phenom build will probably be a little bit faster at gaming and the i7 will be faster at everything else.