Your CPU's default speed is 1913MHz (11.5 x 166). You are running it at 1.53GHz (11.5 x 133) this is like already has been said underclocking. If you have DDR333 (PC2700) or higher rated memory I suggest (if it's running async with the FSB) turning it back to 1:1 FSB:memory ratio and setting up the FSB to 166 at least to get to stock speed first.
When you do that you can start overclocking. You can either use the multiplier or FSB to change the CPU speed. As you only have a KT400 chipset I doubt you will get much higher FSB than 190ish even with good RAM, so you might want to involve a multiplier action when you reach the FSB limit.
go to google and do a search for Prime95. start upping your FSB, then playing a game to see if its stable. When you start crashing in games, lower the FSB about 2-3MHz and run Prime95 on torture mode, you should be able to run at least 12 hours to consider your OC at least somewhat stable. For me 24 hours of Prime95 define a stable overclock. You should keep your eyes on them temps, at full load under prime make sure they don't go much if any over 50C. I would say 53C is the highest you should go. If you see you're braking that mark, try removing your side cover, and consider a better heatsink. I'd recommend ThermalRight SK-7 for a cheap but very good heatsink, and you can select any fan to ur desire, if you don't care about noise much get a Delta/Tornado for best performance, or the Thermaltake smartfan 2 for adjustable but still an awesome fan.
If your temps are much lower than 50C and your OC is not stable you could up the CPU core voltage some, 1.7-1.75 is pretty safe if you have low temps, this will add stability and increase your CPU's headroom for overclocking, although as far as I know an ASUS board doesn't allow over 1.85v I would just want to note that I don't suggest setting it any higher anyway with these chips.
I hope this helps you.
One more thing when you overclock you might have to choose between higher FSB or higher multiplier, although almost always a higher FSB (with even a bit lower CPU speed) will get you better performance in games and applications as it increases memory bandwith, so make sure you get the best FSB:CPU speed ratio for best performance.