For $12,000, those Mac Pro's can come with eight cores of Intel Xeon CPU's (enterprise-grade with extra onboard cache). They can also use up to 32GB of memory, among other nice features. When you need the extra resources or horsepower, this is what you buy.

If your 160GB drive is close to full, then I would look at a Mac with a drive three or four times that size to start with, just so you have tons of room. You probably have tons of crap on the PC that you won't be migrating, though. I'd also suggest not just buying the cheapest iMac -- maybe something in the middle? The 24" models start at 1499 US$, and can do 1920 x 1200 on those ones, so you are covered for 1080p.