When researching desktop graphics cards, I noticed that many nVidia cards support SLI, only the most expensive support triple SLI. However, I did not see any cards that said they support 3-way CrossfireX. I saw a motherboard that supported 4-way CrossfireX, but that did not help me find a list of compatible graphics cards. Are all CrossfireX cards compatible with 3- and 4-way CrossfireX, or are there only a few special cards?
Yes -- all ATI CrossfireX video cards are compatible with 3 and 4 way CrossfireX.
You cannot mix outside the same "family", however, and every article I have ever read on the subject strongly recommends using the same model number. (minor brand differences are OK, as long as it's the same ATI model number.)