nevermind...
As you already have the Xeon 56xx series, your system board is very likely the latest revision.
To be sure, go into F10 Setup, System Info and check the boot block date.
If it shows 01/07/2010 then you are good to go.
The Xeon X56xx series offers much more performance than the E56xx but also needs better cooling (130 vs. 80 watts TDP). You should get two performance heatsinks, otherwise you could end up with your system refusing to boot.
Two quad core Xeon X5687 CPUs are also a good choice, these are a bit cheaper than X5690.