Most of the older slot 1 motherboards have multiplier that maxes out at 8.5, while the celeron 900 uses a multiplier of 9x. Please check the specs of your mainboard before considering just a processor upgrade. If your board can't handle a 9x multiplier, you might be beter off with a Celeron 800 or 850, which use multipliers of 8x and 8.5x respectively.
Also, I agree with most of the previous posts stating that you won't see any real performance gains going from a PIII 733 to a Celeron 900. However, if you might want to consider overclocking a Celeron, as I have. Most of the newer Celerons (800,850, and 900) with the latest steping have consistently been able to overclock beyond 1,000 Mhz. A few have even reached 1,200+ Mhz. For comparison purposes, an overclocked at 1,000 Mhz will perform fairly close to an 866-900 PIII.
As far as an upgrade of the motherboard and CPU goes, you might have to upgrade the power supply as well. AMD processor use a lot of power. I've successfuly been able use a standard 250W power supply with AMD processors up to 850Mhz. Any speed beyond that, or if you have a lot of other devices in your system (multiple hard drives, etc) you'll probably need to upgrade to 300W or 350W.
Hope this helps.