auntjemima is incorrect when it comes to the motherboard specs. That link is for the motherboard that was used in the Optiplex GX270. The GX620 uses the updated version of that motherboard, which supports DDR2 RAM and Socket LGA 775 versions of the Pentium 4 and Pentium D. The best that you can do for that mobo is one of the Presler Pentium D's.
The GX270 and the GX620 are two completely different models, and two completely different CPU platforms. The GX270 uses the FPGA478 socket while the GX620 uses LGA775. Two completely different and incompatible sockets.
By the way, the Foxconn LS-36 is not a motherboard at all - but a PCI riser card instead. It is compatible with the dedicated riser slots on both the GX270 and the GX620 SFF systems.