Get the
M Box with Pro Tools LE. Your OS doesn't matter because the software will include discs for both OS 9 and OS X. If you don't already have 512 MB of RAM, then get it upgraded (that will cost less than $100). The system requirement is for 256 MB, but, trust me, you'll be much happier with 512 MB.
If you're serious, you want Pro Tools. If you need to share files with anyone who's in the business for real, you want Pro Tools.
Get the
M Box. Down the road, you'll be glad you did.