Depends on what OS you have for that x51.
Also depends on who manufactured the card...
SD usually goes up to 2GB. That's no problem for Axims
SDHC goes up to 32GB. I know that 4GB works fine with Axims. Anything above that, use the
SDHC fix.
If you use WM6.1, it can support up to 32GB.
Right now, I have a Axim x51
v running WM6.1 with a 16GB Sandisk SDHC and a 16GB Kingston CF.
The bottom line is 4GB SDHC should be fine. 8GB usually works. Anything above, you decide.
Box seems to know about this better than me, he might chime in...