The PIII 450 is not the same as the 550. The 450 has a locked (on the processor) Clock multiplyer of 4.5. It uses a 100 MHZ bus speed. If you purchase PC133 Ram and if your board supports higher bus speeds you can probebly overclock the bus speed to 112mhz to 125mhz. Though it is better to not overclock at all, the 112mhz bus and below are your "safest". taking the 450 to bus speeds higher without adequate cooling WILL kill the processor.
As I stted before it will depend on your motherboard and what bus speeds it can handle.

Hope this helps

With most video cards you can safely push the 450 to 504 assuming you have adequate cooling on the case, proc, vid, etc.  Won't just heat up the proc, trust me :)

Video cards are VERY prone to overheats, especially when you take their hertz ratings more than about 8-10% over the 66 they're comfy with.

Sharkyextreme and tomshardware are 2 good resources for overclocking.  

I've done it with this proc, TNT2, Abit BH6, stable at 504 for 2+ years now, never a lick of trouble in fact.