Capturing is all in the CPU. The Radeon does not have hardware MPEG or AVI compression despite the fact that an MPEG DEcompressor is built into the card. To capture at 30 fps in multimedia center tell him to set the capture format to either MPEG 1 or MPEG 2. Then under advanced options specify to capture "I frames only". This results in less compressed files, but he should be able to capture at 30 fps (he should be using 29.97 if he's capturing NTSC television) as long as he has at least a 666 MHz Celeron. The faster, the better, obviously.
If he wants to capture to fully compressed MPEG w/IBP compression, then I honestly don't know how fast his comp needs to be. Maybe if he didn't max out the motion prediction slider he could do it with a 600 MHz P3... 1 GHz+ would most likely be better...
If he wants to permanently save stuff for later viewing, he should just record in barely compressed MPEG-1 and then compress it to MPEG-4 using either Flask MPEG or Virtual Dub. MPEG-4 takes a while to compress (DO NOT TRY TO DO IT IN REAL TIME), but it has great quality for its low size (perfect for burning to CD-ROM).