To try and answer your questions one at a time, as far as I know and if someone knows different feel free to comment, you won't have that much look with gma 3100 don't get this confused with gma X3100.
If you want built in graphics go for an older board with gma 950.

There is some support for alc662 but the kexts seem to be hard to get hold of. I used a GA-G31M-S2L for my girl friends mac and put it inside her old G3 case she's pretty chuffed with it. Haven't found a kext for snow leopard yet.

Never had a problem with any of the realtek on board lans every gigabyte board I have tried they have work otb but need driver for bonjour.

From your list I would definatly go for the GIGABYTE GA-EP31-DS3L I think it's the one that will cause you least problems

Thanks for all your replys I just took the opportunity and baught an Intel Core 2 Duo E6600 2.40 GHz & a Gigabyte GA-P31-DS3L for peanuts on ebay - what still leaves out about 40€ - 50€ for RAM. :) - Any suggestions? :D

doesn't matter at all.
just go for at least 1.5gb.