Sounds like you have a 512mb stick combined with a 256 stick (-64mb for shared video memory mayhap?). You can find some DDR ram for decent price. Im not sure but most older emachines run a max of 2gb ram. Your Sempron CPU is terrible however, so its questionable how beneficial 2gb of memory will be.

Did you do as myself and others suggested and go to your msconfig startup tab and start checking off processes?

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1985 CA 300D Turbo , 213K mi