regardless... for 450 you're pretty handcuffed into buying price/performance regardless...
http://uk.pcpartpicker.com/p/3RjAR
That's about as cheap as you're going to get buying quality parts... if you won't be emulating windows for gaming, you could certainly knock the GPU down a few pegs...
I don't run Linux, so I can't officially comment, but I've heard AMD CPUs/GPUs operate more fluently with it, which makes sense from a GPU standpoint using OpenCL
At least from my understanding, nVidia has the best closed source drivers, but AMD has open source drivers that are better than everything else.
The problem is some people say nVidia, others say AMD, so I'm probably completely wrong. But at least from what I've read this seems to be the best conclusion I could draw.