I'd recommend Hallmark too: I use Card Studio 2009. It has decent pictures, is endlessly customisable, you don't have to faff around trying to get the pictures in position and you can upload your own photos too.
Get yourself some decent card (Decadry do some good ones) and away you go. I'd recommend printing the inside of the card on ordinary typing paper and then sticking it in - this is so that if you make a mistake on the inner, you don't have to waste the more expensive card and also there's not a problem with perhaps your printer leaving smudges on the front of the card if the ink isn't completely dry.
Woolwell, Technotiger, northumbria61, exdragon & BT - Thanks very much for all the advise & recommendations for making Greetings Cards. It's a lovely Hobby and for the amount of greetings Cards sent out it could save time and some money searching around the shops for what you want.