My pc:

athlon 3700+ (San Diego)
msi k8t neo2 v2.0
2x512mb ddr pc3200
nvidia 7600gt agp

Is it worth changing the motherboard for an Asus A8N32-SLI Deluxe(100€) and getting a similar graphics card(100€ pci-e)?

I think it would be pointless, you may as well save up for a whole system upgrade to C2D/DDR2/New Gen Gfx.

Not worth it. AGP is dead, and so, for some odd reason, is 939. More RAM might help, but DDR is dead too

If you want PCI-e, then the A8N-SLi Premium is just as good (albeit with the 8x/8x SLi slots) and quiet, and cheap, and i gots one for sale :P

Bottom line - wait.