Yes please turn off caps.
You can get 423 to 478 converter and you should be able to put at least a 2.2ghz with a 400mhz FSB or maybe even a 2.6ghz with the 400 mhz FSB I believe that was the fastest P4 with the 400 mhz FSB.
Upgradeware p478 Socket 423 to 478 Adapter Converter
You can find them on eBay for around $10 to $15 and the P4's are also on eBay for around that same price
Good Luck its always fun messing with old hardware.