Well since you're making the cards physically, all you need is graphics software or photomanipulation software and then print on the cardstock taking into account the paper size, orientation, etc. Some nice free programs are GIMP (Windows and Mac) or Seashore (Mac) unless you want to go all the way with Photoshop.
The picture you put on the card is another thing. If you are hand drawing the picture then you'll need a scanner or a digital camera and then edit the picture with the software (brightening, coloring, fixing in general). If you're going to be drawing the picture with the software (which I don't fully recommend with GIMP/Seashore. The quality is better with Photoshop... that's why I draw by hand, color later) you may need a graphics tablet.
You've got a cool idea with doing the cards by hand but I just use Zazzle.com to put my paintings and such on ready-made greeting cards.