I would not run an overclocked CPU over 55 deg C at max load (video gaming for 20 minutes or running the sisoft sandra CPU burn in test for about 5 cycles). That would be about 50 at idle. Most people prefer to keep their CPUs in the 40's.
It may say 55 but there is so much variation in temp sensors (under socket or on cpu diode), MB scaling factors, and even variation in the actuial sensor characteristics the temps could be 10 degrees higher. Also the cpu temp will go up with room temp come summer so you need some safety margin.
Check the CPU temp once a month to see if the system and/or fans are building up a dust coating, it raises temperatures. With power off, blow the dust off with one of the duster products. Don't forget inside the power supply too. I blasted some dust bunnies out of there you wouldn't beleive!
If you keep the temp in the mid 50's you can keep pushing the CPU speed a little at a time until it won't boot/crashes. At that point you may need to do a cmos reset. If you have an nf2 board nvidia has a utility that lets you overclock from windows so if you go too far and the system crashes you just reboot and your back at the lower bios settings. If you set the bios and it won't boot you have to do a cmos reset which carries the risk of screwing up the bios (see other posts and MB warnings about power being off during cmos resets). Gigabyte also has a windows overclocking tool. In both cases make sure the utility is set so it does not automatically overclock at boot up.
I have my XP2600 Barton just a hair under an XP3000 rating with total stability and max temp under load at 43 deg C. But the Barton has more surface are than the Tbred so it cools better so your resulst will be hotter depending on your fan and case cooling.
Also remember in overclocking if the FSB and memory clock are not the same speed your performance will be lower so try and overclock them to the same amount even if it means keeping the FSB a little lower than is possible. Even cheap 333 RAM seems to overclock nicely at CL2.5
