I have 31" Pro-Comp AT's, TT up front, and I had to trim a good bit off the rubbermaid stuff in the wheelwell, you can see where I trimmed in the first pic. Towards the back of the tire, where the step bar meets up with the fender flare. I had to cut some of that so I could turn without rubbing.
IMHO, 31's look perfect for a close to stock X, it should have came from the factory with these things.
Splashman is right about the tire sizes, my tires measured just over 30.2 inches tall, but believe me, they fill in the wheelwell nicely. I honestly don't know how people get 32's under there and still turn the wheel though
EDIT: forgot to mention I got these Pro-comps in a buy 3 get 1 free deal, plus got free shipping for buying on a weekend from 4wheelparts.com, plus free mounting and balancing on the rims (rims were 100 bucks a piece IIRC). Then after they were delivered took it to a local shop to have them mounted and my old tires disposed of, that cost 25 bucks total.
There are affordable deals out there, you just have to look.
I got new tires and rims for about 800 bucks.
Usually the procomps have their buy 3 get 1 free sale in March, then have another one in october.