My 2009-era MacPro (quad core Nehalem CPU) running Handbrake can transcode 1080i HD MPEG2 to 720p H.264 at roughly 30fps (using all eight hyperthreads), which counts as modestly respectable. I'd imagine a newer Ivy Bridge chip would do even better. I've been pondering replacing a bunch of 2009-era iMacs in our lab with Hackintoshes of some description or another, which seems like the way to have Mac goodness without Mac priciness. (And to buy monitors once, since they're good for multiple generations fo computers.)