Apple is highly overpriced, especially for maming!
I would not pick the Q6600 Quad. It is an old processor (65nm) and it is slower than a duo 3Ghz E8400 in 90% of your work. The E8400 can be overclocked to 4Ghz if you like, and then it really beats the crap out of most quads as long as you don't do 3D rendering all the time.
I would say pick something like this:
A Shuttle XPC Barebone (Easy and small barebones to finish yourself in half an hour)
An Intel Core2Duo E8400
4Gb DDR2-800 2x2Gb SDRAM
Any SATA Hard drive
Any optical drive
Depending on the type of Shuttle, for example an Nvidia 9600GT/9800GT (not all shuttles have a 2 slot wide space for videocards). These cards are not the latest and fastest for 3D gaming, but are very fast in raw pixel handling for 3D modelling with AutoCad etc.
Should be posible for 400 grannies.