I'm not too surprised, I've never seen a real world scenario where moving to 64bit improved performance unless your bottleneck was the fact you were limited to 4GB of ram. Although at least in Windows, as far as I know Windows in 32bit mode can only address ram in 2GB chunks, so that alone could be a silent limitation in large datasets.
Despite, 4GB of ram and more is here to stay and we all need to move onto 64bit. I'm more curious what kind of performance boosts you'll see when you get ahold of those Core i7 based Xens.