any graphics card will work in any system provided it has a pci-e x16 slot.... different chipsets/processors from different manufacturers have no effect whatsoever.
So basically if you use an ati card in an nvidia chipset, there is no performance decrease, and if you use an nvidia card with an ati/amd chipset there is no performance decrease.
There is also no icnrease for matching brands either, anyone that tells you otherwise is bullshitting you.