northwood 512kb cache. prescott 1MB cache.
northwood cooler than prescott.
generally prescott can overclock better than northwood.

to answer the question which is better; actually both same in day to day usage basis, clock to clock comparison.

Added you in the list. Welcome to old school club.

What is your budget?

720p no problem smooth as butter. 1080 then 

Any AGP cards will suit your mobo. You can get HD3850 if are looking for a high end one.

What graphic card are you planning to upgrade? Perhaps you can list down the model and it will be easier for us to help.

None of your choice is good for can play new games
Spend a bit more for HD3650 or HD3450
http://forum.lowyat.net/topic/942444

9600pro from your list if the memory controller is a 128bits model.