If you want to run this chip reliably, I recommend getting a board that specifically provides the 3.6V printed on the chip, rather than 3.3 or 3.45V provided by most PCI boards. As Feipoa has demonstrated, the Cyrix 5x86 chips tend to be pretty voltage sensitive, especially the 120 and 133MHz models.

Also, I should tell that running a 120MHz chip in a PCI motherboard isn't really ideal, because you are running the PCI bus out of spec. It really depends on the board, and what kind of cards you have in there...but generally messing with PCI is not a good idea. I'd leave the 120 for a VLB board, or underclock to 100MHz on a PCI board...or get a motherboard with a PCI divider.

120Mhz is fine as long as you have a board that makes 60Mhz FSB and has an 1:2 PCI divider. The popular UMC boards have these features (Gigabyte GA486AM/S, Shuttle HOT-433, Biostar 8433 for example). The voltage may still may an issue here.

Running 120 MHz at 40x3 on most 486 motherboards can be complicated even if you have a 1:2/3 PCI divider. Aside from the fact that you will be running your PCI bus slow (~27 MHz), many boards will not cope with the fastest cache settings at 40 MHz. In fact, some will only run stable with the slowest cache settings at 40 MHz, which defeats the point of 40 MHz to a large extent. However, if you insist on 40 MHz and the fastest cache settings, double-banked cache of 256 KB seems to work best. Single-banked cache has more trouble with faster cache speed settings.

Sometimes you can source all your PCI cards to run OK at 40 MHz without the divider, however I still ran into long term stability issues, sometimes having to yank out all the cards to get the board to boot again. As was already pointed out, your best bet is to run the Cyrix 5x86-120 at 2x60 with a 1/2 PCI divider. I have found that even with the slowest cache settings at 60 MHz, it makes for a very fast system. Refer to the Ultimate 686 and 486 Benchmark Comparisons for speed comparisons at 60x2 and 40x3 using the Cyrix 5x86.

Best of luck!