its not the cpu, undernourished cpus are usually fine in miners. even single core cpus have been used with no problems (in most miners).

probably a bios limitation. vga cards need a lot of resources, and the mobo/bios can run out unless special settings are used.

see if the bios has something like "above 4g decoding". short version is it increases the range of memory the mobo can assign to the cards, which is likely the issue. that bios setting is fairly recent in consumer mobos as having 4+ video cards wasnt very common back then when those mobos came out.