My car started having boil over issues shortly after replacing the head gasket, despite the temp gauge showing a normal reading right smack in the middle of the gauge, and I thought for sure I had a cracked head or block (and of course was freaking the frick out at that point, lol). Went through all the normals - tested the brand new thermostat, flushed the block and radiator I don't know how many times, new cap, new water pump, etc etc etc. I even have a long thread either here or on Volvospeed concerning it.
To make a long story short, turned out that I had a hairline crack at the base of the threaded part of the resevoir, and I found out that these cars NEED to have pressure in the cooling system in order to circulate properly (went through most of a tube of JBWeld to cover the crack, and the cooling issues instantly went away). The crack in the resevoir bled off the system pressure, thus slowing or stopping the flow of coolant, and how I got the coolant boiling over in the resevoir without an overheating condition being shown on the gauge.
To goodrun: coolant boiling is exactly what it implies - coolant hot enough to boil, and it will be obvious as you'll see the bubbling in the reservoir from the outside, and most likely will hear it as well. In my case, I bought the car with a failed head gasket, and I remember seeing the coolant cap sitting just a bit off level, but I never really thought about it before. When I finally found the crack, the cap was sitting quite a bit more off level, which is what let me to look at the top of the reservoir more closely. I now think that the reservoir developed the crack before I got the car, caused the system to bleed off pressure, slowing the coolant flow, and eventually the car overheated.
From my research here and on other forums, it looks like if your head gasket did in fact blow, chances are real good that it just warped the head which can be fixed with a planing at a machine shop, and the block will be fine. When I tore mine down, it was an obvious head gasket failure, involving cylinders 4 and 5, and the machine shop took the head down to level. Block was fine. Put it all back together, and all is well after finding the cracked bottle and replacing it