Well even with xGrid you need to know some programming. The problem is you can't easily split up tasks like you may think. Only the programmer knows what the tasks are and how they can be split up.
Well you could install some Grid middleware that runs on the Mac, but nothing nearly as helpful as Xgrid and in reality the management overhead would overtake any performance increase.
Of the middleware biggies, Globus Toolkit doesn't seem to work well on Mac though peopel have tried, see this.
I know for a fact neither LCG2.x or gLite run on mac as they are currently only on SLC (scientific linux).
Can't find any ref to UNICORE on mac.
Also, Grid computing gives improvement on tasks that can be broken down into discrete parallel jobs, so it depends what you want to do. In reality I'd think unl;ess there is an Xgrid 2 machine hack it wouldn't be worth your while.