How many photos will a 128mb xd picture card hold on my olympus stylus 800 digital camera?

depends on picture size (both pixels and quality).

My olympus has cam has 128MB card too, and 2288*1712 pics at JPG quality level 9 (out of 10) take up 700-800Kb, so it's like 140 pics. The pic quality is good - can be printed on letter-size paper w/o any visible grain.

The Olympus Stylus is an 8.0-megapixel camera, and so if you have it set so that all your pictures are the highest quality possible (8 megapixels), your magic number is 40.

40 pictures - that's all.

So I suggest setting your camera so that the pictures you're taking are 6 megapixels or less. You only really need 3 megapixels if you're posting a photo on a website. The higher MPs are for good quality 8x10 (or higher) prints. So unless you're printing out all of your photos this big, set your camera to a lower MP level.

At 6 MP you can take 60 pictures with your card.
At 5 MP you can take 74.
At 4 MP you can take 95.
At 3 MP you can take 113.

Have fun - you've got a nice camera there!