Are you using a PC or a MAC?
Pro tools will not make you sound better. Neither will any other program. It has a particular workflow that some people like, and others don't. A large amount of commerical studios use it, so you will be able to take your session files in without the need to consolidate to wav [or another format] files.
You will need to buy the Digi hardware interface for pro tools to run, but that hardware can be used with other software. The software comes with the hardware when you buy it.
To be honest i'm stuck at the moment, because i've used pro tools for a while, its annoying to switch. However in Pro Tools LE (HD is far more expensive),
because of the limited automatic delay compensation, the 32 track (48 with the production bundle) limit, poor summing i'm tempted to switch rather than go HD.
If I could go back i'd probably still get an Mbox and learn pro tools but not upgrade to a 002. I'm not using the pre's on my 002 so its basically a giant dongle for pro tools with a volume knob. Digidesign hardware can operate other DAW's so I'd buy their cheaper interface and another program at the same time, so that when you reach the limit of an LE system you can upgrade to an apogee or rme interface and continue down the pro audio path without having to go HD.
Heres a good article on choosing a DAW
http://www.harmony-central.com/articles/tips/how_to_buy_gear_daws/