the advantages of bands have been talked about soooooooo many times so i dont want to sound like a record, but you should go read some of westsides articles. read the one on accomodating resistance. pretty much (in layman's terms) they make you faster which in turn makes you lift bigger weights. they are a must if you really want to get strong.
For the initial investment,(for bench purposes) start with a pair of mini's and the green bands. Untill you get up super strong(500+ on the bench) these should hold you over. I was just thinking theres over a bazillion exercises you could do with this setup.