Well I really cant tell you, but you'll have to open your case for this one. Is yours a rackmount or just a tower? Sounds like a rackmount. (They're so cool) Well open the case and look at the processor currently installed. Observe whether it is a "Cartridge" or slot processor. It should be black. If it is, is it shaped like a rectangle or a more square-ish shape? Or it could be a "Chip" or Socket processor. The chip should be green. Either way, Remove the heatsink/fan, and remove the processor. Then write down all the numbers written on the processor- It should be alphanumeric.
So, what to buy?
If you have a Black Rectangle Processor, then buy this:
Intel Pentium III 933MHz, Slot 1, 133MHz Bus
If you have a Black Square-ish Processor, then buy this:
Intel Pentium III Xeon 933MHz, Slot 2, 133MHz bus, x,xxxKB cache. (If you have a Xeon it would be best to match cache amounts.)
If you have a Green Chip Processor, then buy this:
Intel Pentium III 933MHz, Socket 370 FC-PGA, 133MHz Bus
Next comes stepping. In order for the processors to work in SMP, they need to have Matched Steppings. Remember when I told you to write all those serial #'s down? Ok, now look for somehting that looks around 5 characters. It should start with SL. Just find another processor with those 5 characters and you have a match. You might even want to contact Gateway and see if they can provide you with a match. Since steppings for OEM processors and Retail processors are totally different, don't try your luck buying every last Pentium III in the store only to find none are matches.
Or you could just buy 2 faster processors, ususally if you buy 2 OEM processors online, they vendor would be nice enough to match them for you, or you could even call them. Remember that OEM processors are bulk packaged and are usually in clear plastic trays. They can just read off the steppings and find one, with any luck.