So your upgrade path is basically throw the thing away...
That looks like a notebook with a stand.
I have one of those G-5's and I Love it!
I have the G5 17" iMac, it's awesome but I would have bought a Mini if it were available. The G5 is nice, but a G4 would have been fine for my needs.
Introduced January 2002, Discontinued July 1, 2004... That's a good run of 2.5 years. Millions are still in use running OSX.
The original price of that iMac was $1295 - $1499 ... current used price $1000 - $1150.
Show me a 3 year old PC that has retained 75% of its value.