I don't have experience, but as far as I know, starcraft is nearly all integer (its the AI, and I don't see anything that would require decimal places....) - therefore, the Cyrix should be faster *gasp*. If you ever want to do anything at all FPU intensive, the P55 will eat it for lunch.
BTW - its really 262 mhz
[EDIT]
I've played starcraft on a large 8 player map with 64mb of RAM. It did slow down a little bit, but when a player joined that had a faster CPU but less ram, the game chocked. Just make sure you have a lot of ram (at least 64mb), and I'd think you'd be fine with the Cyrix....plus it cracks RC5 faster ;-)