so bummed (hard drive)

I installed my new hard drive but it shows up as 31 gigs, not 120 gigs WTF?

someone please help. I so sad. 31 is better than 20... but. :(

the troubleshooting says that some "older" computers arent compatible with newer drives and will only show 32 gig, 64 gig, etc, instead of the full capacity. and that I might have to upgrade my bios. Well uh. (1) what's a bios. (2) I searched apples website and couldnt find anything about bios upgrades to download, etc.

they also said I could use the tool cd to set up an "overlay" on the drive to support it's full capacity? uh? huh? is that wise?

:( I'm wondering whether I can fix this, or whether I should take the whole thing back and just deal with my tiny 20 gig hard drive. :(
maybe I should break down and finally replace my computer. it is kinda old but... :( it's a 500 mhz G4 fyi...

took me so long to back up all my files too. :(