can i use SDHC 2g card in conon a540 ?

because i own a A540.

Well, 2GB is the ceiling for SD cards. Anything above that falls under the domain of high capacity SD cards. So it should work. But you may need a firmware update to handle it. But allow me to make an pitch for using several cards, rather than one large one. Reason being, insurance. Any memory card can fail or become corrupted. And when you're on vacation the last thing you want is to lose the opportunity to take pictures until you can recover that media card with a software utility. If your card goes bad, swapping another one for it at that moment is of advantage so you can keep taking pics. Then, later when you're in front of your PC, you can use a utility like "F-Recovery" to get back your pictures and recover your card.

No need to lose the moment because you relied on one card one when several can solve that problem. At the very least, you want to get more than one of whatever size you settle on.