Clawhammer, NewCastle,Winchester, Venice, San Diego, Manchester and Toledos are all code names for cores, they don't represent a specific CPU.
Right now there are three choices for CPUs:
1. Cheapo: Athlon64 3000+/3200+ (Based on Venice core)
2. Not too cheap, but great performance/overclockability: Opteron s939 model 144 or 146 (Based on San Diego core)
3. Rather expensive, but best overall performance and futureproofness: Athlon64 X2 3800+ or 4400+, 4200+ is not worth it as it has same cache size as the 3800+ (Based on Manchester or Toledo cores repectively. There are many 3800+s based on Toledo cores with only 512KB L2 on each core instead of the full blow 1MB L2 for each core on the 4400+ and 4800+).