As far as the pre-2010 RNS-E is concerned, not all SD cards are compatible. They will NOT work if they are of the SDHC standard. These are becoming increasingly difficult to source at capacities 2gb+. The maximum size possible in non-SDHC cards is 4gb.
If it's a 4gb card you're after, trust me, they're now rarer than rocking horse sh1t encrusted with hen's teeth. 99% will be SDHC.