It doesn't matter what size it is (as long as it doesn't go above 2.2 terabytes,

that's all the X3 bios supports).
As far as brand names go, almost all of them would work, but I would recommend one of the following: Seagate, Hitachi, Maxtor, or Western Digital.
You'll hear a few horror stories about Western Digital, but that happens to rediculously rarely, you don't have to worry about it. If WD is the cheapest one of the four for a particular size, then by all means, buy it. If it catches fire, WD will replace it free of charge. I know that sounds bad, but honestly, it's okay. WD makes an excellant HDD, just as good as anybody else. But, when a company pumps out millions of HDDs, then probably one of them will be put together badly. It can happen with any brand at all.
I personally use a Maxtor 80 gigger, but I wouldn't hesitate to recommend anything else.
BTW, don't steal games: that's not cool. Even though I have a modded XBox, I still don't. XBox Media Center is amazing, though! If you haven't picked that up yet, I highly recommend it! But hey, you didn't come here to be chided by me. Welcome to the modding community!
- Greg
PS: Nice signature...

I was very careful when removing the screws on the bottom and was able to retain my rubber feet, but every one else I know doesn't have feet anymore.