2 different size h/d with RAID?
 

I'm thinking of adding a 40GB H/D to a 80GB H/D using RAID.I don't know if the size difference will affect performance.Is it worth the time? Would it run like a 120GB H/D?(or close to it).

One of the whole purposes for RAID is to utilize multiple disks in various, interesting ways. I cannot recall, off the top of my head, which RAID level allows you to span your data across multiple drives.

The only time you have to worry about congruent drive sized is when you are mirroring partitions (RAID1).

Here is an excellent explanation of RAID. :)

Do you want RAID (essentially more break resistance) or to extend the available disk space?
sounds like you want the latter, in which case research Logical volumes aka LVM: http://tldp.org/HOWTO/LVM-HOWTO/.