like the title says, if a demo runs with a voodoo5, how propable will the game run?

Yes...on the whole. Final engine can be more compatible than demo engine (i.e. UnrealTournament 2003 and Voodoo3/4/5 cards). A good rule is asking here before buying a new game or visiting  this section on it forum

Bye bye

Like some games are really difficult for the VoodooCards, probably preferable to run the demo and try some drivers before to buy a game...and if necessary ask some help

well, i was working on a comparison of different driver sets, and a sort of compatibilty list, but since i'm a student, i cant go out and buy lots of games.... so if demos are representative.
but the website i was writing it for closed down last week....

sometimes, the demo's are surprizing !
once, i tried this demo : Industrial Geant 2
this is a brand-new business simulation game, like a bit SimCity

well, if i believe the box, the game needs Dir8.0 but i ran the demo under DirX7.0a without problem (with a V3 and a V5) !