There have been many discussions here about the benefits and detriments of 64-bit versions of Windows, but suffice to say you won't be 'wasting' your processor by using Vista x86. In fact, your processor being 64-bit only shows true benefits when you're running calculation-intensive applications that have been optimized for 64-bit computing on top of a 64-bit OS. To quantify it, let's say much less than 1%.
The main pro of x64 is support for 4GB+ of RAM and 64-bit apps while the negative is driver support and Vista x64's enforcement of signed drivers. Aside from that it's the minor performance benefit the average user would encounter.
Finally, you will actually receive better performance with XP x64 because XP is less resource-heavy than Vista. Then again, the best performance may come if you run Windows 95. The question is what functionality do you want the OS to possess?
John
here's my question .... i'm about to install windows vista to my machine, i have amd 64 bit processor... i've been reading a lot that the 64 bit version of vista has issues with unsigned drivers, so here's my question...
If i install the 32 bit version of Vista will i waste processing power with my amd 64 bit processor ?
and if so, how much will i waste...?
if not, what are the benefits/cons of running vista 32 bit with amd 64 bit. ??????
is it better to install winxp 64 ???
system specs
amd 64 4000
1.75 GB ram
geforce 8400 Gs
netgear wireless pci wg311t

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic