The 'venice' bit is part of the codename for that particular chip model. They have different names, such as 'Barton Core' etc etc. For upgrading purposes, the important bit is the "s939" - this is the socket shape on the motherboard. You will need another Athlon CPU with this socket configuration if you are keeping the motherboard, PROVIDING the motherboard itself is capable of taking a faster CPU (you'll need to read the motherboard documentation)
One of the better CPUs is a AMD64 4200 Dual Core that runs on a s939, which is available for about £80 from various online retailers, but you will really need to check your motherboard specs first.