What range of sprockets is useful in a road bike cassette? My road bike currently has a 27/39/53 crankset and a 13-26 cassette, both of which need to be replaced.

I occasionally find myself in both extremes—while heading up the very steep hills of the Bay Area the lowest gearing sometimes isn't enough and on much of a downhill grade my legs can't keep up in the highest.

Is it worth going to a wider range on the front or rear?
Ultimately it's a trade-off between a wider range of gears and bigger jumps between those gears.  There will always be uphills that are too steep for your lowest gear and downhills where you spin out.  If you try to fix both problems with a wider range cassette (your triple already has a great range) you may find that you're never quite in the "right" gear for all of the in-between situations.

I'm using a 11-32 cassette on my road bike (with a MTB derailleur for more capacity).  I have plenty of climbing gears but rarely use the small cogs since I spin at around 90rpm.  Like the Bay Area, there are a lot of hill around here.  But I find the jumps between gears too big (either spins too easily or too hard) so am looking for a narrower range cassette.

If you want just a bit more range for climbing, you'll need to jump up to at least a 28-tooth large cog (you'll barely notice the difference going from 26 to 27) which might require a long-cage derailleur.  On the high end, a 12-tooth cog will be a noticeable difference over your 13.  Since you're used to a compact range you probably don't want to go with a much bigger cassette.

Sheldon Brown has a great gearing calculator that you can play with to see what different combinations will give you.