Considering a Mac Pro

I have been looking at buying a Mac for a while and I would like some advice.

I was originally thinking of a 24" iMac (2.93GHz, 4GB Ram, 640GB Hdd) but I seem to have a mental block about paying £1319 (with HE discount) for a Core 2 Duo computer when I recently built a Core 2 Quad PC with high end 2nd user parts for £400. I know there's no comparison as Mac's are better in a number of ways but still....

....so.....I set my sights on a Mac Pro (Quad 2.66GHz, 3GB Ram, 640GB Hdd) at £1595 but is there any need for this kind of computer?

Obviously, it would be great for expansion - I run a web development company and have got 3 x 500GB hard drives which could happily go in this but they could equally go in an external caddy for use with the iMac.

As I said, I just have a mental block about spending this kind of money on a Dual Core computer when I can get a Quad core for a little more. Can someone help?

My uses for the computer would be (apart from general uses), web development, image editing, video editing, publicity design such as flyers, posters, etc using Photoshop.