The best bet is too check the top of the HDD on the Xbox and see what it says, the smallest they made a HDD was 20GB.
Actually, when I first got my 60GB PS within a couple months I removed it and swapped over with a 500GB HDD I was finding myself running out of 'spare' space.
But it wouldn't work installing it on a BC PS3 and switching it back over to a non-BC console seeing as you still have to put the disc in I believe.
(Unless you have a JB or can find one and use a loader.. But meh not likely in your case)
Just get a bigger 360 HDD