You'll need a WD Scorpio HDD, just about any WD Scorpio SATA HDD will be fine but a 5400rpm HDD is preferred (Less heat, and a 7200rpm doesn't perform much (if any) better).
I bought a WD3200BEVT as it was cheaper than a 250GB.
I followed this guide
, it worked flawlessly the first time (My previous attempt (160GB -> 120GB) using the guide in the sticky
worked fine too but it's about a 120GB HDD.
If you want to copy stuff from your old HDD make sure to remove all games first, it'll take ages to copy and some (or all) games won't play from the HDD (I noticed my XBox was louder, removed the game and installed it again and the noise was gone, obviously it played from the DVD ROM).