In my experience, Windows 7 will run on older hardware about as fast as XP does. However, especially with older video cards, you'll probably not be able to do an 3D acceleration, so if you want to watch videos even as simple as youtube, the experience will be much worse than in XP where it'll have the drivers to hand off such stuff to the GPU.
So if you're simply using this as a word processor or a file server, you'll probably be OK. But if you want to use it for internet browsing you should probably keep XP.
Also, 64-bit is entirely dependent on your processor. If your processor is 32-bit, you won't have a choice. And if your processor is 64-bit you'll want to run it that way, even if you have less than 4GB of RAM, contrary to what Lamar says. 64-bit isn't a function of memory (it does control how much memory your system can use, but it's not controlled by memory at all), it's a function of the capability of the CPU.
Personal Experience: I ran Windows 7 on an 8 year old HP laptop running an early Core processor (about 1.8Ghz) and even though Intel would not support drivers for Windows 7, the default drivers included in the OS were very capable. As mentioned earlier, I didn't get 3D acceleration from the integrated graphics, but I still got full/native resolution on the display and the only real problem was watching any sort of video.