Assuming it's a SATA connection just grab a 9.5mm thick 2.5" laptop HD, any size/any speed. I think 500GB @7200rpm is the biggest/fastest available at the moment.
Depends on your Mac, SATA wise that is. Void your warranty? Hardly, the hard drive is, just like RAM, user upgradable/replaceable and hence, doesn't void warranty.
Depending on the age of your MacBook, a 7200RPM drive might actually increase battery life over the 5400RPM one if the old one is from, say, 3 years ago. I have been very happy with my Seagate 320GB drive.
well i got the seagate 500g works great. but i notice i have a sqeek noise i think its coming from one of the fans. It only makes noise when a load is on (thinking) does anyone have any experience with this.