Hi camsam4664. I haven't been n here for quite some time, but I'll try my best to answer. 
As far as I know, the fastest 400 fsb Pentium 4 is the (rare) 2.8 gHz, though if your motherboard supports a higher fsb, ie. 533 or 800 fsb, then faster processors can be used.
 Processor size limits are governed by what the motherboard supports, not by what processor is installed into it.
Any 400 fsb socket 478 pentium 4 processor will work in any 400/533/800 fsb socket 478 motherboard.
Any 533 fsb socket 478 pentium 4 processor will work in any 533/800 socket 478 motherboard.
A 800 fsb socket 478 pentium 4 processor will only work in a 800 fsb socket 478 motherboard.
Hope this helps.