Hi Methanoid,
Yes, on some motherboards, like yours, 2gb of ram works just fine in one slot. Thats great board then allowing you to use 2gb. If i am right it has one pci slot so you could hook it up with some supported pci video card. 2x2gb definitely will not work, period. Its not because of the kernel only, the core of the amithlon is done that way.
Regarding the grub, yes, its very important to tell it your amithlon partition. Do not edit the cfg manually, use template config files and create your new entry upon one of the templates then add and update it. Here is a great tutorial,
http://www.dedoimedo.com/computers/grub-2.html
Regards