Not sure on Segate anymore since I've had the majority(7 of 10) of their desktop drives I bought in the past 3 years for multiple systems die on me. I usually use WD now but there seems to be a compatibility issue with them in these new macbook pros.
I have had several Hitachi Travelstar 7200 RPM drives over the last couple of years and I think they're great (quiet, no noticible vibration, not hotter than stock 5400PRM).
I was a little nervous to buy a Samsung 640GB HDD but Hitachi hadn't come out with anything bigger than 500GB yet and, so far, I love it.