Actually, it does appear you could. The two seem to be from around the same era. Processor is probably swappable, both use DDR RAM. As long as both have matching sockets (likely) as you have open RAM slots (or just remove some from the Dell) you should be fine. Oh, one last thing. You should confirm the processor in the HP is compatible with the Dell's bios. IE, find what the max processor is that was put out in the Dell and be sure it doesn't exceed that. Otherwise it may not POST.