Why exactly would you recommend 64bit over 32? (for killa and grim)
I have used both. 64bit is the more stable of the two. The only reason why one would go for the x86 one is if you have quite a few programs which can't run under 64 bit. Most programs run without any issues. The only two programs which I have which doesn't run under 64 bit is aquamark and the latest leasure suit larry game( for some reason the older ones have no issues but the latest one can't).
On stability the witcher would crash every 5 hours under vista 32 bit but I have only experienced one crash every under x64 and I have finished it 2 times under x64 and before that 2 times under x86. This is due to x64's better memory management.
I would recommended 64bit home premium.