A question of need or want I would say.
This question has been asked a 100 times at Apple Discussions.
Some people have the money to spend without worries and others have a job and need to think economically - Money does not grow on trees you know!
If only
It all depends on how much your willing to spend. If you have the extra monitors (you don't need ACD - normal LCD's are cheaper and great - the ACD is drooling - like a farrari) and keyboard, mouse then you only need to spend on the Mac Pro itself.
If not then decide by comparing the prices of the Mac Pro vs. iMac of the machine that you would want.
The iMac 24" is IMHO a very good machine - an all round package.
Pro user need expandability and power to simulate a protein strand or a complex mathematical equation simulation in the 6000th dimension. Think, Logic Pro, Xgrid services, Server, Fiberchannel cards, ACD 30", multi Operating System Testing. FCP needs additional drives as scratch disk, high end transfers on FW800 (iMac has this also), Pro users are using programs that already utilise multicore processors. For what your going to do - desigining graphics is rarely going to use the other core.
If your going to use your machine to do some serious work then buy it, but if your going to spend alot of money on a Mac Pro because it is the best currently only to show off to friends to get kudos then forget it. Your better off saving the money and take your girlfriend/wife out on a trip abroad and get busy
Now that's investment
As someone said earlier if your asking then you dont need it.