[Cube] Upgrade

[Ted]

I just took my Cube back from my Father .. (Gave him a Mini) The Cube  
has an error on the Hard Drive which is not letting Cube wake up from  
Sleep (even just monitor sleep) and repair disk does not correct.  I  
have OS-X Server running on a mini and would like to make the Cube a  
server and give this mini to my son to play games he is unable too  
with his G3 iBook.  The G3 iBook will go to my Father in-law who is  
having problems with his old Claim Shell iBook.

My question ... which upgrade path should I take with the Cube, if  
any?  My first thoughts were to just upgrade the HD, but noticed a  
lot of different options are available.  Your thoughts???