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I wanted to read about MTRR, because it can improve graphics performance (and I need that because I don't have time to waste writing drivers for closed-standard cards...)
Anyways, the Wiki and most Internet resources are unclear on MTRR. My question is: what exactly is MTRR? How does it work?

Learn to work with official CPU documentation. Your PDF viewer has a search box, where you can enter MTRR. The wiki is no replacement for everything. If you want to learn something, you need to actually make an effort.
In short, MTRRs and later the new format of page table entries let you specify which memory ranges have what caching.
Options include: uncacheable, write combining, write-through, write-back, write protected.