I can answer this first hand, as we have an i850 as well.
We print cards (birthday, thank you's, etc) on card stock all the time, so you will have no issues.
Unlike some other printers, this one does not have a card stock/heavy paper selection, so we just leave it on the plain paper choice (most of the rest are types of photo papers). If you are using something really heavy, try the "envelope" selection before you print.
Should work well, it is a good printer.