Here's what Battlefield 2 looks like on an
Intel Mac
....
....and that's just on a 15" Macbook Pro (which is the equivalent of a First gen iMac Core Duo)
So if you want to run this on an iMac it should have these specs (or better..)
2.0 GHz (1.83 works too but eh...)
1GB RAM or more (the new C2Ds come with 1GB standard)
ATI Radeon X1600 w/128MB VRAM (upgrade to the 256MB if you can afford it, it helps quite a bit.)
(Do not get the $999 iMac if you're planning on running BF2, the Intel GMA card is pretty cruddy...)
Hope that helps you get an idea.
-Josh