I had a similar isue on a car years ago...unluckily mine was acracked cylinder head - minute crack but opened up when the car was really pushed...luckily it was a Mini so the whole head was about $150 fitted!

One thing though...I have a doubt about this diagnosis - excesive presure should be vented via the cap..enough presure to destroy a rad is MAJOR presure so either you have a defective cap (but i would expect hoses to go before a rad) or possibly a blockage somewhere in the engine water ways.....

A leak down test will tell you about a stuffed head/gasket but a simple compression test will also tell you a whole lot if its as bad as would seem!