Doesn't make sense that the cards are running too hot. They aren't particularly hot cards, thus no heatsinks or fans on the cards. If your mobo is overheating, there is some other problem.
Try running with the cover off and a window fan aimed into the box before you buy a lot of case fans. If that doesn't fix the problem, case fans won't.
Again, try moving the cards around to eliminate IRQ conflicts. Install/remove one card at a time to see if a single card is the problem. Run without any cards to make sure the mobo is OK.
If all that still fails, and you still want to use the cards, you might want to consider a different mobo. Since these cards don't use the CPU, you don't need a really fast board, you could get an Intel PIII board including memory & processor for a lot less than 500 pounds.