I heard a lot of complaints on hardware drivers problem with 62bit Windows. For now nVidia haven't even released a good 8800s driver for 32bit Windows. Since i use my computer as a multimedia PC (music, DVD & gaming) i supposed 32bit Vista is good for me for another 5 years before jump to 62bit version.
FWIW, I haven't had any showstopper issues with drivers on Vista 64-bit. I'm also quite certain that the driver situation will improve pretty quickly, since (a) many people want go 4+GB right about now and therefore have a motive to go with 64-bit, (b) 64-bit Vista has all the features of 32-bit Vista, which makes it the first mainstream 64-bit OS around, and (c) Microsoft requires support for both to fully certify drivers as Vista-capable.
If you're interested, here's a more or less full run-down of the issues I've had with Vista (Ultimate, 64-bit):
* No printer driver available for my Epson Stylus Photo 2100P (any Vista version). Solved by using the driver for Windows XP 64-bit (works with no further issues).
* Problem updating nVidia display drivers (version 97.53). Solution: version 100 beta drivers installed and worked fine; update to final version 100 drivers went with no problems.
* smbclient can't connect to Vista share, stopping my automated backup system from working. Not solved; expecting a solution when Samba 4.0 is released. Until then, I'm using Vista's built-in backup scheduler and a USB HDD for my backups.
* Vista backup scheduler can't write to Samba network disk. As above.
* Adobe Creative Suite 2 refused to install into default directory, because it didn't like the (x86) in the path, and failed to launch from the 64-bit program file hierarchy. Solution: enter the install path manually as "C:\Progra~2\Adobe" when installing.
* Firefox 2.0.0.1 renderer glitches when plugin-content is included on pages (the render area starts stuttering vertically, making things unusable). Solution: wait for update to Firefox, use MSIE in the interim.
* Prime95 stress tester did not stress-test; instead, it just quickly filled up my memory and slowed the computer to a crawl as it started swapping manically. Solution: not pursued.
* Certain programs need to be run in Windows XP SP2 compatibility mode (e.g. VtM:Bloodlines.)
* AVG Free does not support 64-bit environments. Solution: bought license for AVG Pro.
That's about it, now. The only really annoying issues I have relate to Samba interoperability; I'm working around them in the interim but would really like to see a solution soon. The only issue specific to the 64-bit version of Vista I've encountered is the Adobe CS2 installer bug, and that had an easy workaround once I found out what it is. IOW, I really don't see any reason not to go with the 64-bit edition if you want to address 4GB of RAM or more now or in a near-term upgrade.