64-bit procs are better than 32-bit ones regardless of the 64-bit capability. Ofcourse they're more expensive, but you're getting your money's worth now, as well as in the future. A 64-bit processor right now will outperform a 32-bit processor even in 32-bit applications. But when 64-bit apps. come out, if you have a 64-bit proc then you'll see incredible performance from it, but if you have a 32-bit processor, then you'll be stuck in the 32-bit age.
They're not that much more expensive anyways...and nothing less will suffice for today's and tomorrow's games.