Ok, even I am getting confused by all the replys.
Its simple.
If you are using 3GB of ram or less stick with 32bit O/S
If you are using 4GB of ram or more you need a 64bit O/S
Between 3 and 4GB its your call, 32bit O/S will not see all of 4GB but you wont loose much.
As for speed. I have some programs that run alot faster under Vista64 than Vista32 and they are 32bit (and one is a game that goes 80% faster on the graphics due to the graphics driver using 64bit calls), I also have some programs that run very slightly slow (at most 1% slower so its not noticeable really).
Drivers were a problem with early 64bit O/S's but Vista64 is becoming main stream and the drivers are now usually available in the same way 32bit drivers are. I don't think I have come across a driver problem for a long time where I could not get hold of Vista64 drivers but a Vista32 one was available.
All current and modern systems are fully 64bit compatible. there may be some really old Intel chips that dont support 64bit O/S's but all Intel, AMD, ATi and nVidia chipsets, processors and graphics cards for the last 2 years have had full support for 64bit O/S.