I would Spend more on the board then the chip. You can get a decent dual core processor for the 775 architecture for $30-$40 dollars. Though if you have an eye to the future, you might want to spend a little more on the mother board. This will enable you to just pop in a new chip if in a year...
I'm pretty sure I'm going to get flamed for this one and I've seen it come up at other places. Basically I'm a linux user. I'm kinda a sucker for the added functionality that compiz adds and am going to want to run gnome-shell when the upcoming gnome 3.0 release comes out.
That having been...