Does your 550 have 512kb L2 or 256kb L2?

If its 512kb then you're not going to be able to break ~600mhz very easily. Either that or your memory timings are too tight for 124mhz.

There are no PCI lock mods for 440bx/lx/etc. Is your Vooodoo 5 AGP or PCI? Either way it shouldnt matter as they have run over 83mhz AGP easily. They also come stock at 166/166 by the way.