Reviews, reviews, reviews...

Question: Ink Cartridge Question and Info on GREAT deal! I just got at Sta

I just got back from Staples with a PIXMA MP640 which I got from their clearance wall for 39.99!!! I was sure I'd get home and look up the reviews and find out it was a lemon - but other than the grumpy gal above - it seems that people have had good luck with it! My question is - will I be able to buy only the cartridges I need as I need them - or when you run out of black - do you have to replace them all anyhow? I anticipate mainly using my printer for printing black ink docs so I just wondered about that.