AOpen S651M motherboard:
1) Processor Support:
Uses Intel processors that fit in a Socket 478 processor socket,http://en.wikipedia.org/wiki/Socket_478
The Processor has the contact pins (478 of them), and the processor socket has the socket holes. (478)
The Maximum FSB supported for the processor is 533MHz.
(Front Side Bus, 533MegaHertz)
Start with the Northwood Core models shown in the link above.
The ones that have a Front Side Bus of 533MT/s, and fit in a Socket 478.
The few shown in the next chart, Prescott (90nm), may not work. The voltage requirement.
2) Ram Memory Support:
DDR Sdram that operates at 333MegaHertz, or commonly known as PC2700.
TOTAL Maximum Amount is 2GB.
There are two ram memory slots. The Maximum Amount for EACH ram memory slot is 1GB.
If you only have one ram memory module ('Stick') place it in Slot 1. (DIMM 1)
This is the ram memory slot that is closest to the Processor.
[ The Processor 'sees' ram memory Slot 1 first ]
Graphics Card Support:
The graphics expansion slot on the motherboard is an AGP slot. (Advanced Graphics Port)
It uses the AGP 4X technology.
Above the 3 white PCI slots is the brown AGP slot.
Use AGP 4X/8X graphics cards.
(These types of AGP graphics cards can be used in an AGP 4X slot, or an AGP 8X slot)
Example of a AGP 4X/8X graphics card,http://www.newegg.com/Product/Product.aspx?Item=N82E16814139053
,and yet another one,http://www.newegg.com/Product/Product.aspx?Item=N82E16814161337
For additional questions, or to have me clarify anything I have stated above, please post in a Comment.