Newf, you're right about the fact that these two are not exactly same day technology - one from the past meets one from the present. However, in practice I saw ASUS, Gigabyte, MSI with mATX boards, based on the 865G chipset, advertised to support both the C2D and AGP. MSI is not my favourite. But for example Gigabyte 8I865GME-775-RH claims to do it (but only modern revisions). AsRock and ECS certianly offers boards as well.
Laserjet, so if you still decide about AGP + C2D, before you jump YOU MUST CHECK compatibilty with C2D of these boards, to be sure you get a new revision board that is compatible with these processors - some of those boards were produced before C2D and do not offer compatibility (power management). So, look for proper revisions. For example Gigabyte that I have mentioned above - only the board with that name and revisions 2.0 or 6.6 support C2D (all support AGP), while revision 1 does not support C2D.
Now, again, I would rather stick to the suggestion of Newf not to take such an old, although chip mobo, but rather modern one with C2D support and with VGA on PCIe, instead of AGP.
Laserjet, however, after all it is up to you and what you want for the future (C2D modern board => PCIe VGA) or how much you're willing to spend (fewer $, "old" 865G chipset that support new C2D + AGP as well as you old DDR"1").