the low power of the atom is hardly worth the increased cost and compromised performance except in only the most niche cases. you cannot play HD video on an atom, let alone high-def flash video, nor can you have background apps running during the playback of SD video. that should be a deal breaker for a lot of otherwise simple multimedia systems.
i think you might be trying to stretch the barebones comparison too far, as the systems are totally different and really can't be compared based on cost. also, that 740G motherboard probably won't support AM3 chips without a BIOS update, and I doubt ECS has gone through that trouble. There is a $55 foxconn that will do it, though. one things is for sure, and i think this is what you were getting at, is that saving ~$60 on an Atom+945 system is not worth it except for only the most basic computing tasks like word processing, file/print serving small workgroups, and web browsing/comms/, and you had better be certain that the PC's "mission" has no chance of changing over the course of it's life, because if there's one thing an Atom cannot do, it would be upgrade.