Well, to start with, it depends what you use yer machine for. You play many games, or do other graphics-intensive stuff? Also, the 550 is a faster processor, and you get an airport card as standard, so it's not only about the graphics card. I don't know what the Taiwan dollar price difference is between what he's willing to pay you and what a new 550 would cost is.
Ask yourself one question (punk ). Is your TiBook slow? does it hamper your work/play? (OK, that's 2 questions) If not, why get a new one?
Basically, the advantages are there if the money seems right to you.
PB G4 500/256/OSX 10.1.3/9.2.2
LaCie Firewire Studiodrive CDRW
PB 1400cs/32/OS 8.5
PB 170/8/OS 7.6
Droppin' science like a cartoon anvil...