Unless you have a specific reason to run 64 bit Windows you are almost always better off running 32. Not only is 64 bit Windows itself larger but since not all software and drivers have been ported to 64 bit you still need a lot of 32 bit libraries to support legacy devices and software. If you have software that requires 64 bit or if your netbook has over 3 GB of RAM you should choose 64 bit otherwise you will be better off with 32.
Is it a 32-bit dual processor (2 32-bit pieces)? If so, the 32-bit is faster. If not, the 64 bit is faster.
Also, you've implied you've already bought the netbook. If you already have it, you have to give your computer major surgery to change the hardware. If your question relates to something being downloaded, you need to check what netbook you have to insure the best performance.
Then if you're downloading some program and you have the options of 32 or 64-bit, take the 64. The program was then designed for your specific hardware. A dual 32-bit does run faster, but from what I researched I didn't see a netbook that had that hardware.
I used 4 bit. It runs like twice as fast now. Thanks!
I have a single core 64 bit processor (Intel Atom)