A faster HD is always a good thing, but its a bit pricey. I think you can get an external HD for only a few $$ more than the upgrade.
More VRAM is great for gaming, motion and if you plan to use an external display.
Anyway, more RAM is the most important you can get, if you can affort it, get atleast 1 GB.
Don't buy your RAM from apple, they are too expensive. I'd recommend Crucial.com, but others think they're too expensive too. Its cheaper than apple and very high quality.