I have Mikrotik on AMD Duron Abit mainboard VIA KT133
1 x Atheros 5212 some no name miniPCI card with adatper.
2 x Atheros cheap GigaByte card
1 x Atheros 5213 older GigaByte card
2 x Dlink VIA network card
all six PCI slot is full in outdoor case work last 4 month in this configuration no problems.
Also use some cheap configuration like VIA chipset for Intel Celeron S478 and work without any problem.
Only have problem with BX mainbaord and some older Pentium 3 but its posible broken mainboard or don't like cheap atheros cards

with one miniPCI card in adapter work now.
Best regrads.