Socket 939 with Dual Channel memory = much better memory bandwidth = much better overall system performance in most apps and especially gaming.
Also never chipsets have improved performance and features e.g. SATA II support, more RAID options than before, higher quality onboard audio and such things.
Actually, AMD are stopping production/EOLing skt939 too to move to AM2 and beyond. Of course, while it means somewhat better prices on skt939 cores for us; we, not being much of a market in international terms, can expect to be getting said skt939 CPUs for quite some time. So, if buying new, maybe skip 754/939 altogether ..depending on intended use/life-span/cost factors, natch.
AM2 is not that useful - all it does is support DDR2 memory for AMD. However, and this is important - your CPU will be supported on the AM2 MB's for a while to come while 939 is going the way of the dodo.
Also AMD's new architecture - K8L (could call it AM3 I suppose) will be supported by AM2 mobo's.