1) During all my years of computer use and tech support, I've never heard of an instance where a clicking hard drive doesn't require drive replacement. With regards to heat, a quick glance at the Seagate website tells me that hard drives should be able to handle temperatures up to 131° F (55° C) - I think if the inside of your PC case ever got that hot, your CPU would have turned into a puddle of silicon-enriched goo.
2)http://www.belarc.com/Drive/smart.html"S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology) is a diagnostic method originally developed by I.B.M. for their mainframe drives to give advanced warning of drive failures. [...] Today all major hard disk drive manufacturers support S.M.A.R.T., including IBM, Western Digital, Quantum, Seagate, and Fujitsu."
FWIW I've never utilised the SMART capabilities of any hard drives, so I have no idea about its effectiveness.
3) I haven't had any problems with Seagate IDEs and SATAs nor Maxtor IDEs. However it seems to me that Maxtor IDEs have a significantly louder hard drive spin-up/spin-down sound.
These days it seems like that if you want a reliable hard drive, you should go for 120GB or less but this is anecdotal evidence AFAIK.