Anyone switching from 32 to 64 or visa versa will almost always state the new install feels snappier (not necessarily measurably faster) than what they had before, but then all new installs feel snappy till you load them up with other applications and AV's etc..
Although, allowing for this, I have myself gone from 32 to 64 to 32 to 64 (I desperately wanted to get my old xbox controller with usb adaptor working after the first upgrade to x64 but there's no singed drivers for that

) on my home pc and changed a couple of work pc's/laptops with vista from 32 to 64 and the 64 I do think seems to run
smoother
So, if you can upgrade for free (eg have the Retail), have all modern hardware which is unlikley to have issues finding signed 64bit drivers and are ready to reinstall anyway (or just for the hell of it), then go for it, but bung in another 2GB to for a pittance to really enjoy the upgrade.