Someone will do the math on this for you. You know, revolutions and streaming at the require stream rates? . . . C'mon boyz - don't let me down here . . you can hear the calculators tapping already, can't yer?
I don't know about calculations. However, I captured all the time to my old 4200 rpm laptop drive. Only a 700 MHz machine. Used a PCMCIA 1394 card. Never dropped a frame. That drive quit (computer came out of standby while in a briefcase and it overheated). I replaced with a newer, faster drive. Makes no difference on capturing, but programs definitely load faster.
Bottom line: For DV, you can get by with just about any modern IDE disk drive; for less compressed formats, you need much faster hard drives (even RAID, if you're doing uncompressed).
I had a very similar laptop to John's and captured video all the time with no problem. Isn't the issue [for DV at least] more sustained data transfer rate than rotational speed? DV is 25mbs or about 3.6MBs. Pretty minimal for even slower hard drives. I might be wrong but hard drive speed was [is] more important for analog capture and (as was pointed out above) uncompressed video.
With the low cost of 7200rpm drives, time spent taking video, time spent editing. . . why would you even consider the extra stress of doubting your capture?