There's nothing wrong with using the 250, but I do wonder what's the problem that keeps it from working with the 80.
If the 80 is on an IDE cable by itself, make sure that the cable is an 80-wire Ultra100 or Ultra133 cable, not the coarse 40-wire Ultra33 cable, and set the drive for Cable Select on its jumper.
If the 80 is on the same IDE cable as the NEC, then I'd change that and put it on its own separate cable, but it sounds like that's what you were doing already.
Anyway, if you end up using the 250, have fun and do be careful about worms. If your WinXP CD is not already equipped with Service Pack 2 built-in, then keep it isolated from networks (wired or wireless), download the whole
Service Pack 2 installer file onto a CD-R or a USB drive, and patch your fresh Windows to SP2 level while it's safely isolated, so it's not as vulnerable to worm attack.
Since your new build supports the Data Execution Prevention feature in WinXP SP2, also
turn on DEP completely and this may help stave off future exploits
🙂