The SD slot on the X50v/X51v can only read the original SD cards (not the new Type 2 ones).
So for that reason you are limited to the 4GB cards.
Make sure you don't buy one of the newer style (SDHC) ones - it needs to be "Standard SD".
The forums at
www.aximsite.com will help you find known working 4GB cards for your Axim.