Yes, most carts are compatible with mk2/mk3. Sometimes, the problem is figuring out how to flash the menu file onto your cart so that it works properly. It depends on the flashing/burning program that is used for your particular gba cart (and whether and how it tries to modify the header information).
All the recent menu releases have had .nds file extensions. This works perfectly for those with neo-carts, but causes trouble for some other people. Sometimes, this can be resolved by renaming the .nds extension to .ds.gba, and disabling the normal loader that gets written to the cart. Other people need a specially compiled menu version with a .gba extension to make it work (cory1492 released that version for the latest menu release)
Another problem is that without a neo/xg cart, you can't flash commercial roms directly to the cart (instead, you must run them from the SD card directly -- compatiblity levels are not very high now, and there are excessive loading times). So this is not the best option out there currently...
BruceG
