My current core on the AMD64 3500+ Socket 939 model cpu is 40C which is to be expected for the older AMD model here. The newer lines of Core 2 Duos and AM2 cpus run cooler. As far as the 305w supply they use someone responded to that quite well on another thread about Dell systems by stating they were better for business then the home user. That can be seen in the thread here at
http://www.computerforum.com/77943-how-good-dell-computers.html where the questions about how good Dells are was asked.
For a company upgrading their productivity, word processing, accounting workstations where the power needs are less and for keeping the overhead down like the electric bill since computers are known for high demand Dell uses a lower end board like a micro atx model in their smaller cases with the small for basic applications and generally light use while saving on the huge demand that would otherwise be seen. For a gamer who pounds on system resources with games that demand memory as well as more power due to higher end video cards a small supply doesn't work. For OCing as well as adding additional hardwares the power needs are much higher. You will also note that the model board in the system there will not likely be seen at newegg or another vendor where system builders expect more.
Your problem seeing system shutoffs and other problems is mainly the demands that gaming places on power. That is one of Dell's budget models best used for email, browsing, and simple desktop apps with some light gaming with the low demand on graphics. Since the demand is greater then the available power the system has no choice but to switch off unfortunately. The one thing you can do there is swap that model in for one of their top of the line models with the support for gaming. That's the drawback of buying prebuilt systems before knowing just what you will need.