If you want something upgradable better go with an Athlon 64 mobo. Something with nForce4 probably. It supports DDR400 (PC3200) DDR, but you may need to change you videocard too as PCI Express is the new video card standard. Anyways a decent PCI ex video is cheaper than its AGP analogue (new technologies
).
As for the mobo brand - ABIT, DFI, EPoX, MSI, ASUS in the such put order are amongst my favourites.
Hope this helps a bit.
Good luck