The heatsink wouldn't just pop off if you'd secured it properly... maybe the heatsink clip was on the edge of the socket clip? That's happened to me more times than I can count (it feels like it's in, it looks like it at first, but you have to push it in further).
But yeah, applying Socket 370/A heatsinks is more painful than applying Socket 478/754/etc. heatsinks. Presumably this is part of why Athlon 64s have a new retention mechanism.
As for temps, if it's stable, it's not too hot.
DEC Pentium X2 5200+ w/ HyperCache (Ezra core)
Asus M7NCD-MAX3 (OPTi Vendetta 82C760)
6x Generic 32MB PC2700 RDRAM (50ns SIMMs)
2x nVidia Millennium X1800 Duo (SLI mode)
12x (daisy-chained) Quantum Medalist 180GXP (w/ separate SCSI-1 adapter)
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