I am running a TigerMP w/ 2X Duron 1GHZ CPU's.. Runs Great. I am using cpufsb to run that at 1150.. like boonesmi said you can run the tbirds too but, i have seen in a couple of reveiws that since the spitfire core is not optimized for smp they don't perform well. The morgan core however is the same as the XP/MP and has the smp optimizations. I say go for the TigerMP and a pair of Morgan Durons. and for $90 bucks for a pair of CPU's you can't beat it. I plan on upgrading the CPU's at some point in the future but, I might just get fatser durons.
The biggest drawback to the dual AMD solution is that you will need to run ECC REG RAM if you will be using more than 2 slots.. 256MB Crucial ECC REG is $100. I bought 4 off these for $31.00 each just before RAM prices went berzerk.