Okay, you can get an 805 Dual core for around $130 at the moment from newegg. Or you can get a 3200 or Opteron 144 for $150-$170.
If you're doing SMP aware applications then the dual core is a decent competitor, if not then i wouldn't touch it with a bargepole. Both options overclock well, but the 805 seems to need an expensive motherboard to do so, while the opteron can be OC'd to hell and back on a $90 motherboard.
I'd go AMD, but that's because the application that needs the most horsepower of all those i use is gaming, where SMP is rather scarce.