You should always upgrade the motherboard and CPU at the same time. You will see almost no advantage to a newer motherboard until you have a processor to go with it. And why AM2+? Always go for the latest socket when upgrading your motherboard, in this case AM3.
I would upgrade the video card and get 2 more gigs of ram. All the video cards around $100 are more than twice as powerful as what you have now. That should hold you for a good while. As for upgrading the rest I'd save up for an i7 system personally.