I found the specs to the motherboard on Gateway's website, looks like they use the same board for some of their computers:
Based on the info from the above link, it's hard to recommend a CPU upgrade when you are limited to 89W and under CPUs. Mostly, because you will not likely see too much of a performance increase going to a new chip and also compatibility may be an issue.
You can get a x2 265 for under $100 on ebay, going from 2.7 to 3.3 Ghz should theoretically give you a CPU performance increase of ~22%. Depending on how CPU intensive the tasks you perform are, you may not even notice a difference. Please tell us what aspects of the computer are "too slow" for you and we should be able to make a better recommendation(s).
Edit: Just some more info I found.
Some people have upgraded to Phenom II CPUs and claim that the board supports up to 99W chips: http://www.tomshardware.com/forum/266637-30-emachines-mcp61pm-support
Just as a alternative, as others have suggested, it would cost more than $100 but you can get a new mobo and CPU which may be a better solution all things considered. Does your current setup have DDR3 or DDR2 memory? A new CPU and motherboard combo on newegg will be ~ $150-$180.