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More info is needed. If it is a PowerMac then you can most likely pull that processor out and put a new one in. Otherwise your luck may not be so great. There are some upgrades for other computer, but the PowerMac is the easiest to upgrade. So if it is a PowerMac, probably yes. If not, then maybe. But I don't think that you can overclock the processor that is in it, but someone else might be able to provide a better answer here.
 
Well, your warranty is over, so you can also try to rearrange the jumpers next to the processor, that could give you an extra 50 mhz, and if you are lucky it will stay stable. There are threads about this at www.xlr8yourmac.com
Else get a new processor upgrade card from Sonnet or the likes.
 
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