Dual 3.4GHz Xeon, will this mean 2x 3.4HT, meaning 4 logical cpu's?
Anyway, BOINC is a good choise, you'll have built-in service-install, multi-cpu-support, multi-project and caching, so don't need to run multiple clients.
As for choise of projects, SETI@home and ClimatePrediction is the released projects, can also run Predictor@home, while Einstein@home will "soon" be released and LHC should start handing out work again any time now.
There are also other projects in development, so your choises will expand, Lattice is one of these already in closed testing.
How you will share between projects, or only aim on one of them is your choise. 🙂
SETI@home "classic" will end "soon", so wouldn't be a long-term-choise.
CPDN "classic" is a bad choise, partially due to can only run on one cpu so must share with other projects, but also since on Intel-cpu's the BOINC-version is faster. Also, 90.3% of all computers is now running the BOINC-version, and new models like the Sulphur Cycle currently in alpha will only be for BOINC.