Firstly I think the FX4100 is a very good chip. Quad Core 3.6GHZ (3.8 with turbo) for under £100 is very good value.
Obviously pairing it with Linux makes it even better
In the past, the NVidia drivers were a doddle to install and worked, whereas ATI was a bit hit and mostly miss. Certainly the latest versions of distro's are much better at sorting this out automatically for you.
I'm running my MythTV box using an ATI card and the Open Source ATI driver that comes with the kernel. Works very well. I'm not bothering with the proprietary ATI driver.
Your motherboard appears to have onboard graphics in the form of an ATI HD3000. I'd stick with this. You'd be hard pressed to notice any difference by adding an fx5200 card. In fact the DX10 compatible HD 3000 is some 9 years newer tech than the fx5200.
Unless you experience some specific problems, I'd stick with what you've got. Even if there are issues, £30 will get you a better nvidia gt 520, or £45 for a gt 430 (slightly older tech than the 520, but higher bandwidth, better for video).