My Dad's currently got a AM3 motherboard with an Athlon II x2 425e 2.7ghz CPU.
He needs something better, partly for X-Plane 10 which I believe can utilise as many cores as are available and partly so that there's sufficient resources for when my Mum RDP's in whilst he's also using the PC (not necessarily when running X-Plane but just running general programs like browsers, organisers, photo/audio editors, CAD and so on).
He's got a rather outdated GPU, so I imagine that will bottleneck X-Plane if he gets a decent CPU, so if he doesn't upgrade the GPU now (which he may not), I imagine that he could easily use 4 or 6 cores for X-Plane and there would still be more than enough for my Mum to RDP in at the same time. All she really does is browse and check her e-mail with Thunderbird and X-Plane is installed on it's own separate drive from everything else, so there's no issue with fighting for disk access. If he does upgrade the GPU at some point, such that X-Plane will run a lot better on all cores, then my Mum can just use her tablet if desperate whilst he's flying.
Now the question I have is whether he should just get a Phenom II X4 (in fact I could give him my X4 955 and buy myself an 8-core AM3+ as I already have an AM3+ motherboard) or a Phenom II X6, or upgrade his motherboard to AM3+ and get an FX-6 or FX-8 CPU? Even secondhand on e-bay the Phenom II's don't seem to be much cheaper than a new FX-6300 3.5GHz for about £83. I think the FX-8350 4.0GHz for £143 is a bit steep for him but he might stretch to the FX-6350 3.9GHz for £100 or FX-8320 3.5GHz for £113.
Obviously the motherboard will cost anywhere between £40-70. He might need a new PSU going from a 45W CPU to a 95W or 125W one but I think his current one might actually be OK already. If not, he's going to have to upgrade it whether he gets a Phenom II or FX CPU anyway.