The eMachines eTower 600is uses an Intel Celeron III processor that operates at a maximum frequency rate of 600MegaHertz. (600MHz)http://www.emachines.com/support/product_support.html?cat=Desktops&subcat=eTower%20Series&model=eTower_600is
The Intel Celeron 600 uses a Socket 370 processor socket.http://en.wikipedia.org/wiki/List_of_Intel_Celeron_microprocessors#.22Coppermine-128.22_.28180_nm.29
There is a locking arm on the side of the Socket 370 processor socket, that is unlocked by easing the arm over to one side to clear a lock tab, then the arm is lifted up to remove the processor.
Example of a Socket 370 processor socket,http://en.wikipedia.org/wiki/Socket_370
The eMachines eTower 600is uses an Intel 810 chipset, for the motherboard chipset.
[Chip and Chipset are slang terms used for Integrated Circuit, or IC]
The Intel 810 chipset, supports processors with a 66MegaHertz Front Side Bus. (66MHz FSB), or 100MHz FSB.
In an additional link, I will give a recommendation of where to buy an Intel Celeron 600, that uses a 66MHz FSB.
I'm not advertising for this place, it's just a place I have used, and have had good results.
Also information on the Intel 810 chipset.
You could also use an Intel Celeron 700MHz processor, but better still you could use an Intel Pentium III 933MHz processor.
Pentium's typically have Two times, the L2 cache that a comparable Celeron has.The more L2 cache a processor has, the faster it's going to process information.
I also know it will work because out in my shop I have an eMachines T1090, that has an Intel 810 chipset, uses a Socket 370 processor socket, and has an Intel Pentium III 933MHz processor in it.
(Made in 2001, and still works good to this day)
The Intel Pentium III 933MHz processor uses a 100MHz FSB.
The Intel 810 chipset supports processors that have either a 66MHz FSB, or a 100MHz FSB.
If you are not familiar with computer hardware technology, then I have just thrown a whole lot of information at you that is unfamiliar.
Should you like me to break down the information, I would be glad to. Just state so in a Comment.