http://digiex.net/guides-reviews/console-guides/xbox-360-guides/3152-hack-250gb-320gb-500gb-sata-harddrive-work-xbox-360-xbox-360-slim.html

Seems to work. Found a compatible drive at the microcenter closest to me this past weekend and I did the most important part today which was getting a 320gb drive to work on my old white unit. After formatting and the xbox wanting me to verify it's serial number, it showed the drive as empty.

I'm currently working on backing up the data from the old 120gb so I can move it over to the 320gb.

I was thinking I'd have to get a slim with a larger drive so I could have one devoted to guitar games and one to all the other games I own, but it looks like the $37 purchase was completely worth it!

I had to leave my system on overnight while I was at work to restore the data from my desktop to the 5400rpm 320gig. When I got home I tried to overwrite the "folder" containing save games to guarantee that nothing went wrong while I was gone, but the program that does that work decided to crap out. I went ahead and hooked everything to the xbox and turned it on and.... bam, there were all my legally purchased XBL games. Started up Marvel vs Capcom 2 and had no problems getting into training mode.

Haven't tried any full disc games yet, so I can't verify that the system a) sees save games properly and b) sees and loads any and all DLC (guitar game tracks) properly. I expect to do that this weekend. I've been really tired this week and thus sleeping a lot.