I need to choose between two of my desktop computers on which to give to my parents. I'm thinking the processor is going to be my biggest decider. I'm leaning towards the Core 2 in the Dell XPS 410 over the Pentium 4 in the Optiplex GX520 even though it's 0.8GHz slower?
Without a doubt go with the Intel® Core™ 2 Duo E6600. The Intel Core 2 Duos were outperforming much faster Intel Pentium processors. As I just said on another thread I remember seeing benchmarks between the Intel Core 2 Duo E6400 at 2.13GHz and the Intel Pentium® Processor Extreme Edition 965 at 3.73GHz. In that comparison the Intel Core 2 Duo E6400 outperformed it in 16 of 20 different benchmarks and was only barely outperformed on the other 4.