mailny the difference is the cache that the CPU has(L1, L2)....the voltage, proc speeds are pretty similar. You are mainly just paying for the extra cache on the 939. Also another difference is the motherboards that support either a 754 or a 939. The 754's usually have just your regular PCI, AGP slots. Where as the 939 has PCIex1 or PCIe x 16. Plus the 939s have newer chipsets..
all in all eventually the world will upgrade to PCI express and DDR2. Agp is out of the picture now, they've made it as fast it can go (66 times 8x , max data rate = 2.1GB/sec) where as PCIe x 16 is 3.2 GB/sec max data rate. I've heard that DDR2 isn't very much of an advantage compared to 184 pin DDR, but soon it will be probably.
so to sum up, if you just want a good system for cheap go with the 754....if you want to have something that will be very good for now and compatiable with new technology down the road get a 939.....