The Intel 945G is the chipset of the motherboard. The on-board graphics for that chipset is the GMA 950. As long as the motherboard has a PCI-Ex16 expansion slot, you can install and use the ATI Radeon X1550 graphics card, however there is no way to combine the performance of the two. Depending on your motherboard, if you install a discrete graphics card, the on-board graphics may automatically be disabled.
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