Jo, You have to be aware; nay, absolutely sure; what type of card you need.
The common varieties are: SM=Smart Media, SD=Secure Digital, CF=Compact Flash, MM=Multimedia Memory, XD, Sony etc.
They are all different though their purpose is the same.
SD (Secure Digital) is postage stamp size and can't be substituted by any other type.
I use SM and have 128MB in the camera most of the time as it covers most events thus minimising swopping memory. For occasions when I may need addition memory I have 2 x 64MB. Why not another 128MB? Well I can remove one 64MB and have it transferred to CD or downloaded via a friend etc while I have the other in the camera.
If you are going abroad check prices there before buying here. A lot cheaper on the Channel Islands for example!