They should be "Type S" xD Cards which are, by definition, 128MB or less.

Yes, you might get lucky and find that a particular "Type M" xD Card works, but it's not promised.

As the xD Cards get larger than 256MB the odds of them working decreases considerably.

NOTE : While 128MB doens't seem like much these days, its important to realize that 1,000+ Programs, Setups, Keymaps, Songs, Quick Access Banks, and Effects that come with the PC3 series only take up a few Megs. Therefore it is unlikely that the average user will
ever exceed their 64MB or 128MB cards.

More importantly, it's so easy to backup the PC to Mac or PC via USB that you may find it unnecessary to even buy an xD Card.