I was running 64-bit ubuntu on my Asus 1201N, which was an N330. My HTPC is also an N330 running 64-bit Windows 7.
Most OEMs will put 32-bit windows on their netbook simply because it's cheaper. It also delineates them from their bigger bretheren. There shouldn't be any reason 64-bit Linux won't like your N450, but if it doesn't, it will let you know very early on (the install disc might not let you install, or may not even boot). If, for some reason, you succeed in installing (some install liveCDs are 32bit, even for 64bit installations, and don't perform any checks), it won't boot, but you will still be able ot boot another liveCD and install a 32-bit linux.