I'd like to buy a mining rig, something like 4 5850 cards, preferably more. I'd rather not build it myself. Are there any vendors who sell such machines at a reasonable price?
If you have a local computer shop near you, you could also order the parts and ask them to mount. There is a wiki entry for mining rigs that has a few standard configurations (including two setups with 5850s).
You might also want to check the Marketplace section of the bitcointalk forums. Sometimes people sell used rigs there.