wide-net resolution

Interested in exploring stock photography & I've just bought a digitial
camera (pentax, 5MP). I'm wondering, if i wanted to start to shoot pics that
have enough quality to go to a majority of the houses that buy digital
images, at what resolution should I shoot that is still somewhat economical
in terms of mem card usage?

On my digicam I have the choice of
640x480
1024x 768
1600x 1200
2048x 1536
2592x 1944
This highest resolution is saved in TIFF format. Anything lower is JPG.
While the obvious answer is likely "shoot everything at the highest
resolution your camera's capable of", I can only put 8 or 9 TIFFS on a
128MB card!
Are lower resolution images (below the 2592x1944) sellable?

What's the market like for much lower-resolution web-only images, in which
case i could store a hundred pics on one card if that's the market i'm
targeting?

Thanks in advance,
Matt