64bit won't be faster. Either version should run fine, but yes drivers from Apple have not been released yet, so you may have to do some tweaking.
It's Windows 7. Apple hasn't released *ANY* drivers, for *ANY* version.
As for 64 vs. 32 for speed, "AMD64", aka "Intel64", aka "Intel EM64T", aka "x64", aka "x86-64", aka "the AMD-created 64-bit extensions for the Intel 32-bit instruction set" is the only instruction set for which the 64-bit variant is inherently faster than the 32-bit variant. This is because the x86-32/IA32 instruction set was resource limited (not enough registers, slow x87 FPU,) and when AMD made their extensions, they doubled the number of registers, and made it so that in 64-bit mode, you *HAVE* to use the much faster SSE FPU. This means that in 64-bit mode, *EVERYTHING* is faster, by about 10-15%. Because the OS is 64-bit native, even if all of your applications are 32-bit, you benefit from all of the background tasks running faster. There is also no speed *HIT* for switching between 32-bit and 64-bit modes to run a 32-bit application.
For most architectures (such as PowerPC,) there is no inherent speed hit for 64-bit, however because you need to transfer twice as much data in 64-bit mode, even when running 32-bit applications (the 32-bit data is 'padded' to be 64-bit,) you do take a minor speed hit. That is one of the reasons the G5's massively faster front side bus was so welcome. It more than made up for this 'side-effect' speed reduction, although running in native 32-bit mode was still a smidgen faster. On the other hand, x86-64's inherent speed boost from the improved running environment means that is *MORE* than makes up for the increased data traffic flow, making 64-bit mode always faster than 32-bit mode.