45 looks and no response, bah.
Start with OWC and check them out ( I think their website is macsales.com), that will give you a base line for price/size and it will tell you exactly what kind of hdd you can plop in there. Most likely you're going to need a 2.5 laptop hdd, I forget what kind of connection though, that's the important thing.
Next hit up newegg and see what kind of deals they have. If you want techrestore.com will take your mac and do all the updating and such...for a price and you can get it back in I think 2 days or so.
http://techrestore.com/xcart/home.php?cat=329
Make sure you back up all your data prior to this adventure, assuming you don't already.
Hope that helps!
Oh yea, as far as making your 120gig an external, just find an enclosure on newegg.com, pop it in, run disk utility and call it a day.