Definately 32-bit. Right now, there are no 64-bit programs, and driver support is terrible for the 64-bit OS, which would mean that it would run even your 32-bit programs, especially games, far worse than the 32-bit version of Windows. The 64-bit version will be the way to go one day, but its not quite that time yet, especially a gaming PC.
When 64bit is normal in computers, and big companies like Dell (cringes) put 64bit windows only on their pc's than a while after that it should be safe to assume that the main bugs have been sorted out.
There is no 64 bit games out now either, the main developers would start to do 64 bit versions, but not all games would until there is a mass market for them.
Ms also do a upgrade to 64bit if you bought it after 2002, where you just pay for pnp.