Hi @bluedog1us ,
As best as I can find out your eMachines desktop has a maximum capability of 8GB of memory.
According to both the [http://www.crucial.com/usa/en/compatible-upgrade-for/eMachines/et1331g|Crucial] memory finder and the [https://www.kingston.com/en/memory/search?devicetype=2&mfr=EMA&line=ET%20Series%20Desktop&model=60107|Kingston] memory finder, your desktop can use either:
1GB x PC2-6400 (DDR2 800) 1.8V 240 pin DIMM memory sticks or
2GB x PC2-6400 (DDR2 800) 1.8V 240 pin DIMM memory sticks.
Depending on the configuration you have now, i.e. is it 4 x 1GB,2 x 2GB or even 1 x 2Gb + 1 x1Gb +1 x 1GB and also what speed the current memory is i.e. is it PC2 5300 (DDR 2 667) for example, will determine what memory you purchase to upgrade it to the maximum of 8GB. If you leave the existing memory in and it is DDR2 667 and add DDR 2 800 then it will operate at the slower speed.
You may also wish to consider replacing the HDD with an SSD and then using the SSD as the OS drive and the HDD as the storage drive if you wish to gain an improvement in performance.