You can't just "change" it - It is what it is. However, you could install an nVidia graphics card and disable the on-board Intel graphics chip. It doesn't "change" anything, it replaces or takes over for the Intel graphics chip.
You'll need to see what type of graphics slot the motherboard has available (PCI-Express or AGP) and check the power output (and available connectors) of the Power Supply. Those two factors will narrow down your new card options.
If you're not sure how to do all that, list the make/model of the PC or motherboard/Power Supply here so someone can check out the specifics and help narrow down your options.
You need to further clarify what you have, and also what you do on the computer. Are you playing games? I agree, Power supplies often get overlooked when it comes to video cards, seeing as your options are affected by the power supply you have. Also PCI Express slots are often blue, or white, or a colour of some sort. AGP slots are almost always brown. That may help when looking inside the case.
------------------------------System 1
PDC E5200|OCZ 4GB DDR2 800MHz|WD 640GB SATA |Seagate 160GB SATA
|Sapphire Radeon HD 4850 512MB|Corsair 650W PSU|GA-EP45-UD3R
Reply to sprucebr1
One last thing that has a tremendous impact on a GPU choice is the resolution of the monitor. Higher resolutions require stronger cards, preferably with more memory, especially if you play games.
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