i know the final release will be available in 32 and 64 bit... but will the beta release both versions too?

Yes, there will be a 64 bit version of the beta.

The beta will be available in 32-bit and 64-bit.Peter Forster, MVP Virtual Machine, Austria

If your hardware supports x64 (as pretty much anything will that you bought within the last year or so) and you have at least 2GB of RAM, I'd highly recommend going for the 64-bit version. You may stumble accross a few driver issues in case you plan to use older devices (printers etc.), but for many devices you'll find a compatible and sufficiently useful built-in device driver that works with x64. Most 32-bit apps don't have an issue at all running on x64 (including MS Office 2007, which is also a32-bit app), however, some actually come with special drivers and some native x64 components to better leverage the benefits of x64 for their use. Beware that you won't really gain much of the benefits without having more than 3GB of memory - however, x64 is about more than just support for more physical memory, it also doubles the width of the registers in the processor and also doubles the number of registers available for the processes themselves - granted, these benefits are only usable by native 64-bit code, which at least the OS executable would be able to leverage, if your typical apps are all 32-bit.In my case, using the same HW and 3 GB of RAM, I've seen tremedous increase in rendering times for Videos (using Pinnacle Studio 10) - this was with Vista x64 and I'm looking forward to upgrade my Video machine soon to Windows 7 x64 for further comparison. Note that other apps - even most Office Apps - also behave less sluggish on x64, compared to running on the same machine with the x86 version of the OS.

Also be aware that if you do any development work that requires the Jet 4 data provider, it isn't included in teh X64 version.