Dell 4550:

BIOS revisions A07 and A08 have problems with firewire cards.  A06 is the recommended version.

P-4 Prescott 1mb cache processors, or "mobile" versions, will not work in the D-4550.  Use P-4 Northwood 512k cache processors.  533MHz FSB 3.06GHz is the best that can be added.




research:
You can use a Northwood, P-4, 533/400MHz, 512k cache and 128k for Celeron.
 
The maximum processor is a P-4, 3.06/533MHz, 512k cache.
 
You cannot use P-4 Prescotts, 1mb cache, or the 256k Celeron-D in the D-4550.
 
http://support.dell.com/support/edocs/systems/dim4550/specs.htm#1101572