Um I could need some advise, please

Yes; stability is equal. In most cases dual core processors outperform dual CPU setups (at the same clock speeds, cache size, and FSB of course).

If you're looking for some benchmarks, here's some Opteron ones.
http://www.tomshardware.com/2005/11/07/single/

Now I realize you're specifically asking for Pentium D benches, but I don't know of an article that specifically pits Pentium D's against 2 Pentium Xeons.

Another option you might consider is the Pentium EE 9xx series or the Pentium D 9xx series. Those are the best of Intel's dual core.