low 30's C would be a great area, low 40C good, 50C acceptable, 60's starts getting into 'too high'. supposedly 85C is about the max the cpu can take.
most will argue that as long as your system is stable and not crashing, then you aren't too hot. this is true since many temp probes don't measure the temp accurately.
right now room temp 27C, cpu temp 29C (been surfing for a couple hours, very little activity). If I run Prime95 temp will go up to 39C or so...
-------------------------
MSI K7N2 Delta Ultra 400 AMD Barton 2500+ oc'd to 3200+ (200x11)=2.21ghz Antec 430W True430 * stock HSF w Arctic Silver III 2x512 Buffalo DDR400 CL 2.5 Winbond Vantec Iceburg RAM heatspreaders Chieftec Dragon server tower w/ 2 intake, 2 exhaust fans Gainward FX5900 Ultra 128mb 400/850 WD 120GB 7200rpm SATA * Windows XP Pro