I'm not a tech person and this is why I'm here. A friend of mine gave me a computer that runs an Intel Core 2 Quad Q9550 and my brother is trying to trade me his computer running an Intel Pentium D 945. Is he being nice or is he trying to cheat me? Which is better...the Pentium D 945 or the Core 2 Quad Q9550?
See following review of 1st generation Core 2 Duo CPUs vs the older Pentium Ds. Intel's quad cores are not part of the review, but the C2D CPUs should give you an idea of how much of an improvement the Core 2 CPUs are compared to the "crap" your brother has.
My favorite site for such things, behardware, has a wonderful overall rating system that averages around 20 benchmarks into an overall rating. The last test they did which involved those two chips gave the core 2 9550 a score of 124. The Pentium D 950, which is actually a slight step up from yours was rated a 42. By their rating, the 9550 is 3 times faster than the Pentium D.
As for whether he is trying to cheat you or not, such a conclusion is premature. There are other factors to consider. For example, he could have a home built machine with a high end PSU, high end board upgradable to core 2, high end GPU, much larger hard drive, DVD or blue ray drive/burner, etc, while your machine could be a $300 OEM machine whos only advantage is the existing cpu.
However, I would not make the trade. The information we do have suggest such a trade would heavily favor your brother. Even if the above scenario is true and things are more even than they appear, there is little evidence you would benefit more from your brothers machine than your current setup.