M$ has already announced that Vista will be much slower on current hardware than XP is. In fact, to run apps at the same speed as you currently run them, you will need 2x the processor power and 4x the ram (they say this is because 64 is 2x 32... dont ask me why that leads to 4x the ram). A lot of this was in a post on this forum a few days ago where the devs talked about the technical aspects of Vista.
First of all, 64 bit computing is NOT faster than 32. It allows you to do math with greater precision and address more memory, but it's not inherently faster. In the real world, it will be faster, because the 64 bit chips are just faster than the 32 bit chips, but not because they are 64 bit.
Also, because the PCI bus is considered "public", and because Windows Vista implements TCPA, any trusted content travelling across the PCI bus must be encrypted and decrypted on each end each time it is moved from one place to another. This will result in substantial slowdowns, and most current hardware really won't be able to handle Vista.
Vista is not about being faster than XP. If you want speed, you'd be best off running Windows 98 probably. My pentium 90 that I had back in the 90's ran 98 a lot faster than a 1 ghz pentium III runs win XP today, and faster than a 4 ghz P3 with 512 MB RAM will run Vista in a year. Each version of windows has more bloat, overhead, eye candy, etc. Hopefully more stability too. 2000/XP are certainly more stable than ME, which was probably the least stable OS M$ released. I'm not convinced XP is more stable than 2K, but it's definitely more stable than the older 95/98/ME line.