504 (or 518 - at 115MHz FSB?) is probably the best you are going to do. The PII 450 has a .25 micron deschutes p2 core, which (I believe) topped out at 450. People used to get Deschutes 300MHz rated chips, and try (often with great success) to overclock them to 450 or 504 using just a decent heatsink/fan, since 450 or 504 was near the top speed that this core would run with sane cooling. Most (but not all) of the 450 rated chips would do the 504. If all else fails you can always engage the turbo feature and get an easy ~465MHz.
About your multiplier question - most of the 450 rated PIIs I have seen were not multiplier locked, as it was at the "top of the food chain" so to speak, and there wasn't much danger of people remarking them and selling them as faster chips, or of overclockers getting something for nothing. If it is unlocked, I suppose you could try for 3.5x133=466 or 4x133=533(if you have a REALLY good one AND great cooling), but your chip may not like to run at 133. If you can get it to run at 466 using the 133MHz FSB, that would be a great overclock, as it would speed up other things, including the memory, as well. Be sure you make sure that your board has a 1/4 PCI divider at the 133MHz FSB speed before you try it.
IF your motherboard has this feature (CHECK THE MANUAL, OR READ BOARDS AND FIND OUT-tech support probably won't tell you, and is likely to mumble something about a voided warranty and an unsupported feature), you can then see if the chip will cooperate. Once you have established that the board has this feature, the only way to find out if the chip will cooperate is to try it.
Good luck,
Nack
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