Using a browser like you usually do at the same time as you're using the Tor browser to look at Silk Road is generally okay. It does, however, make it easier to perform certain identifying attacks against you -- those attacks are based on statistical patterns when you have a view of a large portion of a given network (like a large ISP might), as well as access to all the data traveling through the network at any given time.These attacks are very complex and difficult to perform, and it's extremely unlikely that anyone would bother just to bust you with a gram of cocaine or whatever your individual pleasure happens to be.Basically, don't worry about it. If you're a vendor or otherwise a person of interest, however, you probably want to avoid this. But if you're just buying stuff, it's nothing to worry about.