I have a 1GB card, and it's big enough to hold the important parts of the GoodNES set (clean ROMs and hacks/translations) with room to spare. If I need more space, I can always delete the ROMs that will never run on the PowerPak (i.e. games/demos that use more than 512K of either PRG or CHR), or the vast majority of the \HACKS folder.
I use a real (AFAIK) Sandisk 1GB and get a "Card not ready" error which flickers rapidly when I start up on an old Famicom, I have to either power on 3-4 times before it starts or hold A while starting up and release. On an AV Famicom I get no errors though.