If Windows 7 will install on a Dimension 2350, it will install on a Dimension 3100/E310.
I'm going to put the 2gb memory maximum theory to the test.The chipset will support up to 8gb.
E-310 motherboard $25, CPU $30, Sata 500GB hdd $60, PSU 430W $42, Win7 $70 (full).
The E310 is standard ATX.
"The E310 is a simple black BTX minitower... ."