I can't tell you the best performing ones or anything like that but remember that the speed isn't of any great benefit on the Ax as it works at it's own pace.
A faster one will obviously work faster in a card reader.
CF, pretty much unlimited capacity as far as I know, only limited by what is available and how much you want to spend.
SD, I think is 4Gig.
SDHC only 4Gig if you are using WM5 with the SDHC patch.
SDHC doesn't work natively with the AX and WM5.
If you flash to WM6 then you have complete SDHC compatibility.
There is a list of compatible cards
here.