I am using an MSI mATX board now with an Asus 128 Nvidia AGP & Athlon XP1500. I am looking to upgrade to a A64 - Sempron 64 to start.
There are some mATX 754 boards that will take the Sempron & they have AGP (so I could use my Asus card) - but it will stop with the Sempron. These boards so far as I see mostly have the VIA 800 chipset.
There are 939 boards (like the Microstar) that use NVidia - thus the 1000 spd bus - and the built in 6100 graphics. They also use the PCI 16 for a vid upgrade - and some will take the Sempron - and also the Athlon 64.
What I want to do if the 6100 is up to par with my Asus is to start with a Sempron on that board - and upgrade later to the better Athlon and a new PCI 16 graphics card. I do not do any heavy gaming - just MS Office, web, and simple games (like Whatword, Zuma).
Does that soumd like a good plan?