Hi Tao. Two most important things to upgrade are RAM and disk. Which one it depends on how you use your Mac, but if you can upgrade both, do it. Your laptop model can have as much as 8 GB of RAM (2 x 4GB). Increasing it helps a lot and it allows you to have more apps open and/or more tabs in your web browser, etc. Replacing your old HDD with an SSD will give you instant speed increase that will shock you. Absolutely recommended. And you can even keep the old HDD and use it instead of the built-in DVD drive (that I hardly ever used). Good luck.
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