I have a 5 mega pixel camera and only print 10 x 15 cm. I don’t do a lot of cropping either.
If I need to save memory card space and want to shoot at a lower resolution, how low could I go before the print quality will deteriorate? I also have a choice between ‘Fine’ and ‘Standard’ Quality. Should I stick to ‘Fine’ which also takes up more space, but the quality is higher.
My choices are 2560x1920; 2304x1728; 2048x1536; 1600x1200; 1280x960; 640x480
Quality ‘Fine’ or ‘Standard’

10 x 15 cm (4x6" if I've done the conversion correctly) - 1mpx (1280x960) should be OK and standard compression would probably be acceptable to you.

but...

First, surely this isn't too tough to try youself. Why not take some shots at the 2 compression levels and possible resolution (12 shots) and have a look?

and second, although obviously I don't know your circumstances, memory is really cheap these days.

This may be false economy i.m.o. Perhaps get a Hitachi microdrive and shoot for the highest quality? Its so cheap to get a microdrive these days. I recommend a recent version Hitachi brand 1gb or 2gb microdrive.

There are more variables: 100ppi with a high quality RIP looks great, 200ppi with a crummy printing system looks horrid. There is more: for example, how sharp is your lens? What is your subject? What type of printer? What RIP or software?

Please do take the time to try some comparison shots, and do let us know what you find? Sometimes there are some surprises. Personally, I only shoot RAW.

Wishing You Great Light!
Paul