Only Dell can tell you the answer to this question. You must ask Dell what processors are compatible with THEIR BIOS. Then, you can select from those.
An upgrade processor must be socket chipset, AND BIOS COMPATIBLE.
And, this is only possible if your existing processor does not use a BGA socket.
Al is correct; unless the BIOS has support for a particular processor, it could be incorrectly initialized (or not at all) - and this can cause all sorts of other problems.