You should be fine with the stock Dell heatsink. In fact, I bought a Precision 390 about a year ago with the least expensive processor available from Dell and then swapped it for a QX6700. Also, at least in my system, the CPU socket is mounted at a 45 degree angle relative to the general orientation of the motherboard and I am not so sure that the placement of the post holes is anything like standard. Trying to fit an off-the-shelf heatsink in there might be a bit of a challange. The stock one is quite large and seems to do the job. Just make sure to use high-quality thermal paste and you should be fine. My system has shown no ailments from the upgrade and it runs 24/7 with a good amount of load most of the time.