"A 3 mp image can be 500kb to 1.5mb. Depends on a whole bunch of variables, but lets say they average 1 megabyte each. Your minimal 128mb card will hold about 250 pictures"

Looks like you did the calculation for a 256mb card. A 128mb card will hold about 125 pictures.

It also depends on whether the camera has the ability to compress the files before storing on the card.

I have a Canon SLR - Only 3megapixel. 5 choices of mode to shoot in.

1. RAW (TIFF)
2. Full frame CCD high res
3. Full frame CCD low res
4. Half frame CCD high res
5. Half frame CCD low res

CCD is there own format. Direct upload into Photoshop and then save in any format you wish.
Comes with a 256mb card. This holds, respectively, 400, 800, 1600, 1600, 3200.

The picture below is in mode 3 (lowest resolution 1.5mp). I use that mode for nearly everything. You can print photo quality up to 24x16 from that.
Obviously I reduced the first pic below to 1/4 size to fit. The 14x10 is beneath it. (The full size file is only 254k uncompressed - 160k compressed on the camera card)