I don't trust any temperature reading software with a Brisbane CPU. The BIOS should give you a fair idea of the temperature though. The BIOS loads the CPU at about 60-80%. My X2 4000+ Brisbane read about 23*C in the BIOS with a GeminII and dual 120mm 80cfm fans. That seems a bit low as the room was probably 21-22*C. My 4200+ Windsor read 58*C at 100% load in Orthos on PC ProbeII with a stock cooler, which is pretty high. The Brisbanes run cooler, but I bet his 46*C he's reading in the BIOS is about right for a 60-80% load, maybe a bit on the high side.
I'd really recommend waiting until you can afford the $30 ($20 + shipping) for a decent aftermarket cooler. Once you get that you can overclock properly to 2.8-3.1GHz. Maybe take a look around the for sale section of this web site, see if there are any good deals on coolers. In the mean time, read up on overclocking and your CPU and motherboard.
If you really want to get started now, ok, but lock the CPU voltage at 1.30v and don't increase it. Set the CPU multiplier at 10x. Set the memory to DDR2 667. Set the timings to 4-4-4-12-24-2T. Set the memory voltage to 2.1v. Set the HT multiplier to 5x. Try increasing the reference clock to 220MHz. Run Orthos blend test for 2 minutes. If you can make it past that, reboot and increase the reference clock to 230MHz. Repeat Orthos test, if you pass, reboot and set the reference clock to 235MHz. Repeat Orthos test, if you pass, reboot and set the reference clock to 240MHz. Now try running Orthos overnight. Come back and let us know if you were able to get that far.