The NF500 is based off of the Poweredge 2950 (source: here). As you currently have a Xeon 54xx series, your NF500 (not sure if this applies to all NF500's) is based off of the 2950 III (this version added support for the Xeon 5200 and 5400 series (45nm processors)).

So with that info, I went to www.dell.com, then small business, electronics and accessories, Parts for your Dell, Poweredge and then the 2950 III. This got me here for a list of processors that Dell offers. You can check what's on that list and then buy from Dell (should get the warranty of the system or 90 days, whichever is longer), or you can shop elsewhere with the info about which processor(s) will work.

Note: Intel sometimes updates a processor with a newer 'stepping'. Dell's bioses aren't known for being the most flexible about accepting newer processor steppings, so if the processor you bought elsewhere doesn't work (usually on post you get a message about having an unsupported processor), you may need to look for an equivalent with a different stepping. Here is an example of a processor with 2 steppings; C0 and E0.

Thank you very much for your help, Dev Mgr.

One more thing: do you think power dissipation might be an issue? The original processors are 80 watts rated, and faster CPUs are 120w and even 150W, also, I guess if the processor has a faster FSB it will just simply work a little bit slower, at the original 5405 speed, right?

Thanks again, man

From what I've seen in 2950's I think all the heatsinks have always been the same, even going back to the "2950-I".

A bit of digging got me to these vendors (first, second, third) offering 2950 heatsinks and none of them mention a wattage or limitation, so I think it's safe to assume that this one heatsink that Dell has been using can accomodate all Xeons (that are certified for the 2950/NF500).

Perfect, Dev Mgr, very helpful... Thanks again.