AMD64 systems run fine in 32-bit mode.  You will get a small increment of capacity by running the system in 64-bit mode, but that may or may not be visible, it depends a lot on what you're doing and how the rest of the system stacks up.
I have been running a AMD64 system in 64-bit mode (Ubuntu AMD64 build at the moment) for a couple of years.  There are a few headaches (a lot of proprietary software isn't 64-bit yet, for instance, Flash being the worst culprit).
If you're planning to run all FLOSS, then something like Ubuntu's AMD64 build is a great way to go.  If you want proprietary software to work, then I'd recommend sticking with a 32-bit build of whatever distro you prefer.

For a laptop the biggest issue is going to be heat and battery life, so you'll want to make absolutely sure that the laptop has an effective low-power mode, and that it's fully supported by whatever Linux distro you choose.  Some of the lowest power laptops are AMD64 based, but not all 64-bit distros support the AMD (or Intel) dynamic power conserving features (speedstep, etc...).

==Joseph++

Nathan England wrote:
> I'm considering some new systems and wondering whether or not to go 64 bit.
> Is there really any speed difference? The only system I know of is a friends 
> and he runs it in 32 bit mode. Are there problems with running the systems in 
> 64 bit?
> 
> nathan
> 
> I'm thinking specifically a laptop...
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