OEM built machines are often better than home-built counterparts, predominantly because OEMs understand how to balance all the various components to get better overall performance. 64-bit probably helps a bit, but not usually by a significant amount.
This is probably true on the hardware side, and definitely in the performance vs. price ratio in my experience. But in terms of software, no way, all the crap that is installed on PC's nowadays (to keep cost of hardware down presumably) is crazy. First thing
I do when I get a machine from an OEM is wipe it, much much faster without all the stuff they install.