Hi,
The Mac Pro will beat out the iMac.
The Mac Pro's CPU's are Xeons and they are considered to be for highend development. Programs like Lightwave, Finalcut Pro, Adobe CS3 Applications all have the ability to function with faster CPU's.
The real answer will come about by how you will use the computer.
Are you going to be doing any professional rendering or high end video editing? If so, the Mac Pro is going to bes the iMac in CPU processing.
If you are only going to dabble in video and use the machine primarly for homework, internet, games etc. The iMac is more suitable.
For the specifics of the GPU Cards. There are plenty of reviews on the cards you mentioned that will answer your question. Games demand more from the GPU than the cpu these days and an iMac with a better GPU than a Mac Pro will look better for games. But never assume that because a computer with a better GPU than another computer is more powerful.
Then again if you just want to just have the Mac Pro for bragging purposes, that works too!