When we look at the project's own stats, sorted on RAC, we see:

1. A fairly anonymous Intel on top (a Genuine Intel® CPU @2.30GHz [Family 6, Model 45, Stepping 5] with 32 threads, aka a dual Xeon E5 2680);
2. An Intel® Core™ i7-3770 CPU @3.40GHz [Family 6, Model 58, Stepping 9] with 8 threads;
3. An AMD Engineering Sample [Family 16 Model 9 Stepping 1] with 24 processors, aka a dual Opteron 6174;
4. Another Intel® Core™ i7-3770 CPU @3.40GHz [Family 6, Model 58, Stepping 9] with 8 threads;
5. An Intel® Xeon® CPU E5645 @2.40GHz [Family 6 Model 44 Stepping 2] with 24 threads, aka a dual Xeon E5645 system.

All-time best Constellation@Home sysems are:
1. An Intel® Core™ i7-2600 CPU @3.40GHz [Family 6 Model 42 Stepping 7] with 8 threads;
2. An Intel® Core™ i7 CPU 970 @3.20GHz [Family 6 Model 44 Stepping 2] with 12 threads;
3. Another Intel® Core™ i7-2600 CPU @3.40GHz [Family 6 Model 42 Stepping 7] with 8 threads;
4. An Intel® Xeon® CPU E5620 @2.40GHz [Family 6 Model 44 Stepping 2] with 16 threads, so a dual system too;
5. Another Intel® Core™ i7 CPU 970 @3.20GHz [Family 6 Model 44 Stepping 2] with 12 threads.
Best AMD CPU is here a AMD FX™-8350 Eight-Core Processor [Family 21 Model 2 Stepping 0] at #32...

Using WuProp, the FX™-8350 is indeed AMD's weapon of choice as nobody has yet put his or her 64-core quad Opteron system to the battle, and the better AMD and Intel cores perform rather equal at 600-900 credits per core per day.

The message seems to be: bring as many modern cores as you can, the faster the better (now why doesn't that come as a surprize? Though I'd rather find that a Power6 chip does the trick)