CHDK does not care if a card is SD or SDHC, but some old cams don't support these cards.
It's just cards > 4 Gb that can be difficult, but only if you need to autoboot from them
Your card should be fine for
A590, just remember that if you format it in the camera,
it will be formatted as FAT32, which is OK if you want to start CHDK with the 'firm update'
method.
If you want CHDK to boot every time you start the camera, you will need to format it with
FAT16; you can use CardTricks to do this easily if you've got Windows and a card reader.
wim
