The dell 420 takes 4x AGP cards so don't insert an 8x agp card!
The system is designed to use dual P3 cpus, a 450MHz CPU may be a P2 or a P3 but the FSB of a 450MHz chip is supposed to be 100MHz. The dell usually runs at an fsb of 133. You may be able to lower this in the bios though.
If you only have one cpu inserted, it must be in the first socket & the second socket should be disabled in the bios
I don't know if the board will take P700 rimms or not. I do know it can support a max of 512MB on a single module, 2GB total memory & must have a minimum total of 128MB of ram.
The chances are it won't support P700 but if it does, make sure you don't put mixed types in obviously especially as Rimms are dual channel.
The ram that came with the system should be ECC Ram so if you use non-ecc you should disable ECC checking if possible.
If the system is just beeping at boot up with no display, it usually indicates a memory problem (most likely) or a video problem. If I were you, I would put all the original components back in the board, get that working again, then replace the components one at a time. You may need to clear the Bios each time though