Your best bet is the p3-450, it will run at 133bus, 600mhz 90% of the time without a problem. Also its pre-coppermine, so its slot-1, though it may be difficult to find. But as long as you have many choices between 100 and 133, like 110, 115, etc, then any chip should give you at least 10% increase.

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1999 MB C230 Kompressor Elegance (200k km)
1994 Volvo 945 (330k km)