GMA950 is a chipset... Ultimately you need slots on your motherboard to support add on video cards. The Geforce 5600 is from the way way back, 1st gen DX9 cards. I'm not even sure if there is a PCI-E variant of that card.
Either way, you need to know what slots you have, and what slots you want to use (PCI-E, AGP, etc).
The 5600 were originally released as AGPs only, but some models were updated to include an AGP to PCI-E bridge, see this from Wikipedia on the FX series:
The GeForce FX line moved to PCI Express in early 2004 with a number of models, including the PCX 5300, PCX 5750, PCX 5900 and PCX 5950. These cards were largely the same as their AGP predecessors with similar model numbers. To operate on the PCIe bus, an AGP-to-PCIe "HSI bridge" chip on the video card converted the PCIe signals into AGP signals for the GPU."