Just as everyone else has said, don't bother. The 3400+ was always close to it even in Clawhammer form, and the Newcastle outclocked it by 200MHz (at the cost of 512K L2 cache). That Newcastle will also outperform the 3500+ (despite the 3400+ having only single channel memory operation), so you can go that route if you don't mind Socket 754.
Also, I haven't seen any SFF 940 barebones setups. The demand wouldn't seem to be there. Its brief stay in the consumer market was quickly ended with the introduction of the 939 FX-53 (maybe just 939 in general, or possibly the 3400+, though the latter could be a little debatable), anyway.