Depends what you want. iMacs in the past would have left you hanging, if you wanted to upgrade. But, the 24" has

- a socketed intel CPU, so you could always upgrade.
- apparently a socketed mobile GPU (iMacs are mobile chipsets, i.e. basically a macbook pro).

I'd go for a Mac Pro, if you already have a sizeable investment in a nice screen that you could hook up your Mac Pro up to.

If you have any more specific questions, please ask... we're hear to help !