xxrayeyes, yes! The server 64-bit chip is already on the market (Opteron) and is very impressive in benchmarks. The client 64-bit chip, Athlon64, has an introduction date set for September 30. The benefits will first accrue most to people who have large sets of data to manipulate in DRAM - this means complex simulations and 64-bit optimized games. It is likely that Windows64 will not be finalized by September 30, it will probably be in Beta stage then. 64-bit Linux is already available, and 64-bit Windows should be finalized for holiday sales.
If you need a new PC right now then get a lower priced AMD machine with the intention of upgrading to 64-bit next year. You can buy an intermediate machine for about $500 if you reuse your monitor, or simply upgrade your motherboard and/or processor for less. Let us know more precisely your needs and we can give more precise advice.