more video memory means that it wont use your ram as much to store testures and i have never heard of motherboard incompatability other than not having the right slot for the card you want

http://en.wikipedia.org/wiki/Expansi...slot_standards

this is the best link i can find to explain the different types, mostly your gonna focus on AGP or PCI-E x16 or x1 for video, they do make PCI cards but will be slower when compared to the earlier two with PCI-E being the newest and fastest so pretty much if your card has the same slot as your mobo your good to go