Newcastle
Newcastle is one of the older AMD Athlon 64 cores, being brought to the socket 939 family from socket 754. Newcastle chips are based on the 130nm process, and have the largest power consumption and heat dump overall next to the Clawhammer core. They've got the normal Athlon 64 cache size of 512KB.
Newcastle is availible in chips such as the 3000+, 3200+, 3500+, and 3800+. It's a dated core and as far as overclockability goes, don't expect much more than 400MHz on air. I really would stay away from this core as I don't think there's any difference in price between it and the more power conservative 90nm cores.
Venice
The Venice is notorious for it's ridiculous power and voltage conservation, reaching clock speeds as high as 3GHz with little overall vcore increase. The Venice uses an improved intergrated memory controller which can account for all 4 DIMM slots running at 400MHz. Previous memory controllers would underclock the RAM to 333MHz if all four slots were in use.
The Venice specifically fixes the silicon problem found in the Winchester and can handle higher clock speeds. It's also the first AMD core to add SSE3 instruction sets, the third iteration of the SSE instruction set for the IA-32 architecture. It is a SIMD instruction set. If you are purchasing a midrange processor, get a Venice based chip.
Note: Some Motherboards require an updated BIOS in order to use 90nm cores.