For everyday tasks such as web browsing and e-mail i doubt you will notice any speed increase over i386. One of the reasons i use x64 is to take advantage of all system memory.
As to the drivers,they are referring to Lan drivers,i would just install Testing and see what works!.
In the past there was no native plug-ins for flash and Java for x64 now there is, here's a link:
http://labs.adobe.com/downloads/flashplayer10.html
For Java i use the 'Icedtea' plug-in from the Debian repositories.
Also if you use the x64 version you will be running a kernel compiled specifically for that architecture.
Go for the install(x64!) and post back how you get on.
Bare in mind these are just my opinions.....
Regards.
The trooper.