But you also need to realize that with Macs, the type of RAM isn't the only important factor. Macs tend to be picky with CAS timings, so it's generally easier to buy a set of RAM modules that are guaranteed to work (which really isn't any more expensive) than to gamble with modules engineered for generic PCs. Several memory manufacturers sell Mac-specific modules for that reason.
Also, being Xeon workstations, Mac Pros are spec'd to use ECC memory modules, although the unregistered, non-ECC ones in your Newegg list may work. It's still a gamble, though.
Like mentioned before, OWC (
http://www.macsales.com) is a great place to buy guaranteed Mac memory, as is Trans Int'l (
http://www.transintl.com). They're both cheaper than Crucial and work just as well.