There is generally no problem mixing video card and CPU brands for single card operations. With two cards, depends on what the mobo supports.
AMD CPUs generally provide more value in the low budget category while Intel CPUs are generally recommended when spending more money on a system . The dividing line seems to be around $500 - $600 - less and AMD generally gives the better value. Although Intel has come out with some low in brands lately to try to lower the thresshold.
For a general review of nVidia vs. AMD/ATI video cards, see this web page:http://www.upgradevideocards.com/designers.html
You can also see a lot of "model" builds on this page - see the stickees at the top. I don't know if they are current but they give you an idea to start with and would allow comparing some Intel and AMD systems.http://www.tomshardware.com/forum/forum-31.html