Is this your sole xbox 360 you plan on purchasing and since you're interested in modding realize before investing the money the LIVE ban bat is now wielded. So if you're interested in the nice feature's of LIVE and running backups the only option is have 2 360's. Also I'm reading that some games will force updates to play, so i'm guessing this might be implemented in popular releases like madden 08 and halo 3. This is where the second unmodded 360 comes handy.
I haven't modded mine but bought the sata card anyways...$13 off newegg so no biggie. I'm not sure what drive you'll get, either samsung, hitachi, or unfirmwareable benq because I have no clue if MS rolled out more drives of a particular type. I know for sure I have a samsung and probably the ms25 since I bought it in Apr 2006.