filling overflow - head gasket?

[bustedplastic]

So i really got out and got to pound on my quad last weekend.  Problem im seeing is the overflow tank was at the low mark when it was cold, but after really running it, the bottle filled up, overflowed, and it bubblesd like crazy.  Thing is, the antifreeze isnt even hot.  After reading posts this sounds like a head gasket issue, am i right?  Is this bad, like "replace immediately before riding again" bad?  And what should i do to prevent this?  Thicker head gasket, stud kit?  Take the cylinder off, sell it, and go back to stock bore??

Thanks in advance...

[tonyrt44]

head gasket is blown allright