I haven't played Dragon Age, but my 2.93ghz iMac with the 4850 runs other 3D games (like Left4Dead and Orange Box) beautifully. I can't imagine your machine would have problems with Dragon Age. That said, I'd probably go for an i5 or i7 machine instead of the Core2Duo.
Of course you'll get even better performance if you get a PC, but then you're stuck with Windows (ugh). Me, I'd rather take a few FPS hits and have an iMac, on which I can run both Windows and OS X, than have a dedicated gaming PC.
The requirements for Dragon's Age are much higher than Steam based games (L4D and Orange Box)
Dragon's Age:
VISTA MINIMUM SPECS
OS: Windows Vista with SP1
CPU: Intel Core 2 (or equivalent) running at 1.6Ghz or greater
RAM: 1.5 GB or more
VIDEO: ATI Radeon X1550 256MB or greater
NVIDIA GeForce 7600 GT 256MB or greater
DVD ROM (Physical copy)
20 GB HD space
Left for Dead:
OS: Windows XP, Vista, Vista64
CPU: 3.0 GHz P4, Dual Core 2.0 or AMD64X2 (or higher)
RAM: 1 GB for XP / 2GB for Vista
Disc drive: DVD-ROM Drive
Hard drive: At least 7.5 GB of free space
Video: Direct X 9 compatible video card - Video card must be 128 MB or more and should be a DirectX 9-compatible with support for pixel shader 2.0 (ATI 9600 or high. Or, Nvidia 6600 or higher)
Sound: Direct X9.0c compatible sound card
Plus the poster already has a Macbook Pro