The E6850 will be considerably faster than the X3210 in most situations due to the 866 MHz gap. There are only a few hypothetical situations where you can benefit from the additional 2 cores in the X3210.

It's hard enough to design an application to get good performance out of two cores, let alone four. You may only experience a benefit when loading the system heavily with multiple intensive tasks. This will uncover bottlenecks elsewhere, such as the FSB, memory speed, disk speed etc.

The E6850 is a better overall choice.