What do you use that PC for?
I guess it's gaming.
What games?
My opinion would be to get a new motherboard with, say, a 7600, and then buy another 7600 for SLI when you need/want it.
If you keep that motherboard, you will be stuck with AGP - you'll need a new motherboard sooner or later, so why not now.
Just my $0.02.