There is not that much unused land around Cal Expo. The overflow parking area next to the Capital City Freeway is probably where they are thinking the arena could go, but then what do you use for parking when the main parking lots along Exposition Blvd get filled up? A parking structure would have to be part of the plan.
I would also hope that they build some 'light rail' into the plans for this new arena - wherever it may end up. Many folks have a sort of cowboy mentality about their cars & trucks - that they're never going to degrade themselves using 'public transportation.' But when public transportation is convenient - i.e. cheaper and more efficient than driving in gridlock - even cowboys find a way to adapt. Meanwhile there are plenty of folks who will use light rail if/when it's there.
I don't think noise will be a problem with an arena at Cal Expo - such as in the case of the amphitheater that used to be there. And a new arena would breed new life into the Cal Expo area - which is fairly close to downtown and just across the river from midtown. With a downtown arena clearly not in the plans, I think the Cal Expo plan is the next best thing. But, it's going to require:
a) an well-designed expansion of light rail
b) some improvements to the traffic problem that already exists (ideas, anyone?)
c) a new high capacity parking structure at Cal Expo, and
d) later event times