This processor isn't the best for overclocking but here goes.
Yes, the processor can be overclocked: (Which speed, I have no idea)
Your motherboard is also a determining factor to your overclock, some boards can handle a good overclock, some cannot. Also, you will want a good heatsink and some good thermal compound Arctic Silver 5 is the most common for overclockers (Antec Silver 5 compound is Arctic Silver 5 just in a different container). As far as what steps you need to take to overclock this processor, here's a breakdown of it.
Once you have the above mentioned items; you are now ready to proceed to warning #1 of overclocking: If you cannot afford to replace the parts, then you shouldn't be overclocking. (This is unless you know what you are doing)
First you must enter into your BIOS. You will find a section where it lists your CPU Multiplier. It's probably something like 12x multipler now since your RAM FSB is 133mhz as that processor is a 266mhz FSB processor.
From here, depending on your processor, and your motherboard, you have 3 ways to overclock.
1: You can increase your FSB (You must have ram that can perform above it's suggested operating speed)
2: You can increase your multiplier.
3: You can increase both your multiplier and your FSB.
Don't push it very high as your Athlon XP 1900 doesn't have much room to overclock. It's also good to take a look at your processor temperature in your hardware monitor section of the BIOS and see what it's idle temps are. Most overclockers calculate their temps in Celsius. So if you see someone post their processor temps, you will usually see something like 40c or 50c. The Athlon XPs usually idled anywhere from 30c to 60c depending on their cooling. I had my Athlon XP 2600 Mobile as low as 36c stable.
I hope this helped a bit.
Here's a great site for overclocking information: www.hardocp.com is the mainpage, and the forums are where you can ask questions www.hardforum.com
I'm also a member of that site as well. If you have any questions or for me to clarify what I've mention, just post back in this thread. I'll monitor it when I can.
That processor is a poor candidate for overclocking. The fastest speed that can be acheived is about XP 2200.
1) The multiplier is locked so you have to overclock the front side bus (FSB).
2) FSB overclocking is best done with a MB that lets you set the RAM, FSB and PCI clocks independently. Check the biostar manual to see if it allows this.
3) The CPU itself runs hot and its heat will go up rapidly as you increase the clock speed. 2200 is about the best you can get.
4) It is possible you will have to do a CMOS reset to clear any too high settings. With an older board like yours it is possible to wipe the BIOS if not extremely careful about removing all power from the Computer before doing a CMOS reset. (unplug and let bleed down for 5 minutes before CMOS reset. Follow instructions in manual and on biostar web site exactly.)
If the PCI and RAM clocks can not be set independently you end up clocking your RAM and add in cards too fast which can cause problems and in the case of the add in cards damage.
If your PCI speed goes up with the FSB speed setting don't bother to try and over clock this CPU, the risk is not worth the improvement.
If you must do this download sisoft sandra lite to use as a burn in tool.
1) Adjust FSB in bios 2-5% at a time depending on what the bios allows.
2) Boot and see if windows loads. If it doesn't you went too far. BAck it off. If windows loads run sandra in burn in mode to exercse the CPU arithmetic. Watch the CPU temp. for 20 minutes while running burn in. 55 deg C is max acceptable. IF it goes over that back off the speed.
Be aware that booting with the speed too high can cause windows to corrupt the HD when it crashes. So back up all data, liscences, have drivers ready, etc in case you have to reload.
Now ask yourself is all this trouble worth a few percent improvement?