[link|http://www.pbs.org/cringely/pulpit/pulpit20050127.html|Dethroning King Gillette\nIs iPod the Razor or the Blade?]
H.264 decoding performance is almost entirely dependent on the speed of the processor's Altivec unit, which benchmarks show to be comparable in both chips at similar clock speeds. So if a 1.25 GHz G5 has enough grunt to decode a 720p-24 stream, then so does a Mac Mini.
Only issue I see with this is a lot of sets will not accept a 720p signal. They expect the signal provider to reformat 720p to 1080i or 480p.