Basically, I think there's an agreement that the 2.33Ghz upgrade for 250$ is pointless for MOST people. of course some need it, but probably not you.
Graphics card, IMO, is a must upgrade. Not just for games - for everything. This screen is huge, and it needs a decent video card. Core Animation in leopard will need as much juice as possible.
Max the RAM to 2GB. 3GB is pointless at this point due to pricing.
you MIGHT consider upgrading the hard drive IF you have tons of stuff to store, but if you already have an external drive, you don't need the upgrade.