In terms of overall gaming performance, the Intel Core 2 Quad Q9650 3.0GHz is massively better than the Intel Xeon Processor X5450 when it comes to running the latest games. This also means it will be less likely to bottleneck more powerful GPUs, allowing them to achieve more of their gaming performance potential.
The Core 2 Quad was released less than a year after the Xeon Processor X5450, and so they are likely to have similar levels of support, and similarly optimized performance when running the latest games.
The Xeon Processor X5450 and the Core 2 Quad both have 4 cores, which is not likely to be a limiting factor for gaming.
More important for gaming than the number of cores and threads is the clock rate. Problematically, unless the two CPUs are from the same family, this can only serve as a general guide and nothing like an exact comparison, because the clock cycles per instruction (CPI) will vary so much.
The Xeon Processor X5450 and Core 2 Quad are not from the same family of CPUs, so their clock speeds are by no means directly comparable. Bear in mind, then, that while the Xeon Processor X5450 and the Core 2 Quad both have the same clock frequency, this is by no means an indicator that the two CPUs will provide the same level of performance. As such, we need to look elsewhere for more reliable comparisons.
Aside from the clock rate, the next-most important CPU features for PC game performance are L2 and L3 cache size. Faster than RAM, the more cache available, the more data that can be stored for lightning-fast retrieval. L1 Cache is not usually an issue anymore for gaming, with most high-end CPUs eking out about the same L1 performance, and L2 is more important than L3 - but L3 is still important if you want to reach the highest levels of performance. Bear in mind that although it is better to have a larger cache, the larger it is, the higher the latency, so a balance has to be struck.
The Xeon Processor X5450 and the Core 2 Quad have the same L2 cache size, and neither CPU appears to have an L3 cache. They even have the same L1 cache size, so are identical in terms of cache size.
The maximum Thermal Design Power is the power in Watts that the CPU will consume in the worst case scenario. The lithography is the semiconductor manufacturing technology being used to create the CPU - the smaller this is, the more transistors that can be fit into the CPU, and the closer the connections. For both the lithography and the TDP, it is the lower the better, because a lower number means a lower amount of power is necessary to run the CPU, and consequently a lower amount of heat is produced.
The Core 2 Quad has a 25 Watt lower Maximum TDP than the Xeon Processor X5450 (though they were created with the same size 45 nm manufacturing technology). What this means is the Core 2 Quad will consume significantly less power and consequently produce less heat, enabling more prolonged computational tasks with fewer adverse effects. This will lower your yearly electricity bill significantly, as well as prevent you from having to invest in extra cooling mechanisms (unless you overclock).