After a motherboard to suit these specs on my friend's PC, she has:
Intel ® Pentium 4 CPU 2.40Ghz
256MB DDR RAM
To be honest, she's not a heavy PC user at all and the person who uses it most in the house is the 7 y/o kid, so the cheaper the better! Onboard stuff would be nice too, since I don't think she has separate sound or graphics.