Roughly the same, I’d say. I have the 2.0 Ghz Core Duo iMac, a 3.0Ghz P4, and a newer AMD 64 Dual-Core (4800, 2.4 Ghz).
The iMac is in the middle, speed wise, when running games on Windows (I’m using BootCamp, of course, not an emulator or virtual machine). Surprisingly, Apple actually put a pretty good video card in the iMacs this time; I’ve got the default one (not the extra VRAM), and it still runs Oblivion quite well. The AMD dual-core trounces them both, but it has a very high-end video card, which will skew the results.
The only benchmark I’ve played with gives me ludicrous results, probably because of the different architectures. Where lower numbers are better, the P4 = 70, the iMac = 15, and the AMD = 11. That’s the right order, but not the right order of magnitude, the iMac and AMD machines aren’t THAT much faster.
Note that my P4 is slower than yours, and my iMac faster than your proposed one, so I’d call it about a wash. It will depend on the application. The big advantage of the Mac is that it also runs Mac OS X, useful for video/photo/music work. The downside of the Mac is that the current BootCamp sound driver (I haven’t checked for a new one in a month or so) can’t shut off the speakers when a headphone is inserted, so sound comes from both sources. This isn’t a hardware limitation, since Mac OS X gets it right, and it’s a known problem that they’ll presumedly fix before BootCamp is non-beta.