I'm using a SanDisk (big name) microSDHC 4 gb card. Reads all 4 gb and stores ~4 gb tested on my P168+.
I've heard that some of the older models will only read up to 4 gb, so if you run an 8, you'll only see 4. I've also heard on some of the newer 168C that it will handle (or at least recognize) 8 gb.