I just got a G6 Lite for under $90 shipped from realhotstuff.com.
It's the same exact size as the DS Lite's GBA slot cover so it does not stick out of the system. It comes with interchangeable cards to match the DS color (although the white is not a perfect match, but it's VERY close)
It's a 4gigabit (512megabyte) card and can work with a passme device or a flashed DS. It has the compatibility of the M3 (I've actually read its better.. they're from the same company) and has been very very nice so far. It also plays GB, GBC, NES, PCE, and SMS games (I may have missed a system). I haven't messed with this area much, but I'll probably use it soon.
You can't upgrade the memory, but you don't need to go buying flash memory and USB readers. I'm happy with it, and it's cheaper than the DS-Xtreme. Plus, you can keep a DS game in the DS slot to have an extra game with you.