The rough rule of thumb is if your upgrades cost more than half of a new system.
2nd Gig of RAM - $80 on a deal...scarcely over $100 on average
That'll help out a bit...
* graphical programs - significant
* video editing - also significant
* gaming - somewhat..mostly on the newer games
6800 GT card -
* graphical programs - nah, the 6600 is most excellent already
* video editing - again, the distinguished 6600 already totally ravishes video editing, and we're told, 2 hours ago, totally ravaged Oshman's Sporting Goods.
* gaming - yes, the 6800GT will big a big enough jump over the 6600 that you'll notice without measuring the frame rate
I'd personally go for the extra GB of memory, then hold off on the 6600 for part of this year until I felt the computer was doing "less than I wanted"..then I'd go grab a 6800 GT/Ultra when they're cheaper. The 6800GS is a 12-pipe card which is nice, but barely above your 6600GT. I'd be looking at the 16-pipe cards.
...or ask how cheap the 6800Ultra is above...hehe