The two machines seem very similar in terms of per-CPU-core processing power. The X6800 was also bought in 2007 and is the same generation CPU (Conroe) as the Xeons in the Mac Pro. In terms of Geekbench numbers, the single thread performances are almost identical - for example, image compress/decompress is 2107/1782 on the Mac Pro and 2111/1879 Mpixels/sec on the Hackintosh; sharpen/blur is 6129/7715 and 6153/7740 respectively. For multithreaded benchmarks, the Mac Pro has four times the number of processing cores, so one would expect it to outperform by a factor of four. This bears out: the multithreaded sharpen/blur is 48122/60809 on the Mac Pro as opposed to 12389/15232 on the Hackintosh.
The Hackintosh's memory benchmarks are about 25% better when compared with the Mac Pro. Average memory/stream scores for the former are 2790/2287 as opposed to 2200/1818 for the latter. Mostly this is due to the memory speed difference (DDR2 800 rather than 667) I think.
The overall Geekbench score is 4105 for the Hackintosh and 10519 for the Mac Pro. At work, I do a lot of technical computing involving parallelism so multithreaded performance is important, but for day to day and personal tasks both machines are similarly (and plenty) capable.