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I would agree with what others have said about the PSU, most pre-built systems use poor quality PSUs which are only just capable of running the hardware supplied, so any graphics card would have to be a low power one (one that doesn't require supplementary PCI-e power connectors). The cases also tend to have poor airflow, and not much room for a graphics card. Best option would be to self build, you'll be able to get more performance for your money.