The 5150 is nominally a 120W head, and mine kicks out almost bang on 120W through the clean channel. To be safe, you should aim above this by 'some margin' - people will probably kick in with decent technical answers, but if you can get 200W of power handling you'll be safe when cranked (besides, the amp will blow your ears through earplugs above about 7/10 on the post gain! These f***ers are LOUD!!).
My cab has the G12K-100s (100W handling) and the EVM-12L Blacks (300W handling, but count as 100W because that's all the G12K-s can take!) in an x-pattern, one pair (series with each other) is parallel to the other pair, and I have one of each type of speaker in each 'series' pair. I don't have time to link you to a site now for working out power handling based on series/parallel wiring (assuming all the speakers are the same impedance), but you should get familiar with this if you intend to swap out speakers from any cab! You'd be absolutely fine with a 5150 cab and you can pick these up fairly cheap (and they sound pretty good!).
Roo