We just finished development on our game, Critter Chaos, and all we used was a Mac Mini.
We didn't really have a big budget, so we just needed the cheapest mac available.
The only time I noticed it being too slow was after a few hours working in both Photoshop and XCode. However, I wasn't using both programs at the same time too often though...
I have a lesser spec Mac mini: Core 2 Duo 1.83, 1GB RAM, 80GB hard drive. It does slow down quite bit. But I'm using Corona SDK at the moment so I develop on my much better spec windows machine and just build on the Mac mini. I have a drop box folder setup so I don't have to worry about transferring data.
