https://support.hp.com/us-en/product/compaq-sg3-100-desktop-pc-series/4169543/model/4229013/document/c02556928 shows a pair of PCIe X1 slots so expansion is a dead end.
You'll have to replace this PC to game at the levels you want.
Be sure to look over https://www.reddit.com/r/PCMasterRace/wiki/builds as maybe you can recycle your keyboard, mouse, monitor and hard drive.

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