What exactly does the BSOD say?
There are many possible causes for a BSOD and the text of the BSOD usually indicates what triggered it - it might also be an idea to provide details on how you're doing the backup - is it a backup utility, is it W2K3's native NTBackup?
I would guess that the chances of your problem actually being caused by the NAS at extremely low - as far as Windows is concerned, it's just another network share out there, pretty much like your Buffalo NAS was - and that it is more likely to be the backup software or perhaps your network card hardware/drivers.
For what it's worth - I backup my Server 2K3 to my NAS (DNS-323) and have never had it bluescreen - I've used Symantec's BackupExec and EMC Retrospect