There are certain things for the home that I don't really like purchasing. Sheets would be included in that group so I usually wait for my mom to buy them for me. I can always tell the difference between the one's she buys and the one's I buy. I am concerned with the price so I generally purchase the cheapest thing I can find. She's willing to pay a little more to get a higher thread count.
In general the higher the thread count, the better quality sheet you get. But did you know that some manufacturers are stretching the truth a bit when it comes to thread count? Good Housekeeping did an investigation and found that some manufacturers are doubling the count when making sheets with 2-ply threads. A 2-ply thread is 2 threads wrapped together to make a single thread. But that doesn't mean that manufacturers that use 2-ply thread can suddenly call 400 count sheets 800 count.
This whole thing gets really long and complicated, but I found this:
This site gives lots of helpful information on selecting quality sheets. In fact, they believe that thread count isn't the most important factor in selecting your sheets!