Should not matter which drive you install the OSs on.

If I'm right what you want is 2 OSs on 2 HDs

If you already have vista set up then what you need to do is make sure its not un upgrade version.

This is kinda of backwards project. HD A=250gb DH B=320GB

On HD A you have Vista
ON HD B u need XP

It sounds like you are having trouble with the xp cd and not the hardware.

If thats the case proceed with this:

Once you get XP to install finish the set up. At this point XP is the only thing that will boot without Vista. Once you have XP set up and squared away.

Get you XP disk out and load in into Recovery Console with Vista and Choose the option to fix you boot file.

Once that is done Vista will load and all you have to do then is Download VistaBootPRO and let it run over all drives. (you can find more info on this prog else where) Then restart your PC and you should have Vista Boot Manager with XP and Vista.