This is a huge can of worms, but I think that erikhertzel has jumped the gun on his recommendation.
First, we need to know when it's slowing down. What sort of programs are running when you see the slowdown, what sort of programs you run for general use. On most PCs you will see a performance bottleneck in one of three areas:
1. CPU utilization
2. Memory utilization
3. Disk utilization
If you play lots of 3D games, then your video card can also be a 4th bottleneck area. Also, if your memory utilization is high you will usually see a high degree of disk utilization as the system swaps out pages to the swapfile.
If you are running general office/business apps and doing some web surfing, your system should be more than adequate and I would look for a possible configuration issue or malware infection. If you are running high-end imaging software, video production, etc, then you may need more memory or faster disks.
Your best bet is to open task manager (hit ctrl-alt-delete and then click 'task manager') and watch your CPU or memory utilization via the 'performance' tab during normal use. Pay special attention during the slowdowns that you experience and note any usage spikes or sustained periods of high utilization. Use the 'processes' tab to determine which applications are eating up the most memory and CPU cycles. Report the results back here, and we can offer an educated opinion on what the problem might be. Running a virus/adware/malware scan wouldn't hurt either.
Moving to Windows XP x64 Edition won't do anything to improve your performance, unless you have some 64-bit applications that you want to run that can take advantage of it. On 32-bit applications the performance is nearly identical, but drivers for the x64 edition are much more scarce than for 32-bit Windows. "Upgrading" to a Vista beta isn't recommended, since it's a beta. That means that not only is it potentially unstable and buggy, but it also is a bit of a resource hog (performance tuning is usually the last thing done to an OS before it ships). Adding that to it's already higher system requirements would likely make your experience even more frustrating.