I've built two computers myself (with all the upgrades they should probably count as six. ), but I bought my current one from a firm that assembles them to order.
I had an interesting problem with the last upgrade I did myself, I replaced the motherboard and CPU, after reassembling the basic components, I started the computer for the first time. Well I tried to, all I got was a beep code, which was translated by the manual into a message about wrong CPU speeds. Since I had never been in the BIOS, I knew that could not be. After a weekend of trials and frustration, I brought the computer to the shop where I did buy all my hardware at that time. It took them several days to figure it out, which made me feel a little bit less stupid. They even tried to build the same computer with the same hardware from their stock to eliminate all chance of a faulty part. The problem was that the RAM-modules were incompatible with the motherboard, even though type, speed and everything were according to specification, that motherboard had been tested by ASUS with only a few specific RAM-modules and it didn't work with most standard RAM. So you should download the manual from the ASUS-homepage and check if the RAM you're planning to buy is on the compatibility list in the manual. Here's the download page for the P8P67 LE: http://www.asus.com/product.aspx?P_ID=TzGzLFtd2HBF7vc5 Nowadays they even indicate part numbers and in some case even chip numbers of compatible RAM.
Concerning the case, I've got the Thermaltake Shark and I have to say that I'm very impressed by the finishing. It's well thought through, especially those little details like the way drives are mounted and the most important thing: no sharp edges inside the case.