I don't like Proposition 8, and as a straight person I am fine with gay marriage. How it harms anyone's marriage, I don't know.
The ability of a State Supreme court to to hear an amendment to the state is non-existent, just as the United States Supreme Court can't challenge an amendment to the United States Constitution. Amendments limit courts' powers, just as courts can limit other branches of governments, by striking down laws.
Because courts can't challenge amendments to their respective constitutions, the arguments in the Proposition 8 challenge evolve around what are basically technicalities around jurisdiction. In essence, do the people or strictly the legislature have the authority to do what they were trying to do (legally).
Overturning Proposition 8
Overturning Proposition 8 will simply mean the court considers the people to have overstepped their authority. It is entirely conceivable that something similar (anti-gay marriage) would come out of the legislature, in the event it was overturned.
Consequences
Such a decision will be viewed many scholars and layman alike--not universally--as bad law. There could well be a push to impeach some or all of the justices on the court, for what many may view as overstepping their authority.
Upholding Proposition 8
This is more than likely to happen, since the basis for overruling it (you need more than wanting equality) is fairly contrived. There is a fair amount of speculation that the challenge will go to the federal courts. This would be a good bet, since they could have authority and form a legal basis for overruling Proposition 8.