Zebo, how do Opterons consume less power?? The 146 has exactly the same wattage as the 3200 venice, i.e. 67W. Check the AMD quick reference guide if you dont believe me. I think you are comparing to the Newcastle which consumed 89W but who's buying those these days?
MCochrane, if overclocking is really not important to you, then get the Athlon. These Opterons are getting harder to come by (in the UK certainly) so you could be denying some poor overclocker of his dream processor. As Mucker has already said, it is a crime not to overclock the Opteron
Ignore all the advice to go cheap. Its good advice if you are overclocking as the biggest bang for buck is achieved on the cheapest processors like the Opteron 144 or A64 3000. As you are not overclocking, buy the fastest processor you can afford as this will future-proof your system for longer.
As warranty is an important concern for you, choose your mobo carefully if you go for the Opteron because many mfrs wont give technical support. Gigabyte do apparently.