Windows 7, 32 vs 64 bit

I don't see a benefit to going 64 bit in your case. You have 4GB, which is usually plenty for most apps, except large image editing on Photoshop, which likes more memory to go fast. You're plinking on the internet, so no need for more.

I do recommend using an SSD for your system drive. Just do it, and you will be amazed at the speedup for any machine. I use a 180gb drive for the PC I am using now, and it astonished me with the speedup. Boot in 30 seconds, not 3 minutes. Every operation speeds up. Smaller SSDs, 120-180GB, are quite cheap and the best speedup I know of for any PC. This does require that you clone your system partition to the SSD, which is a technical challenge for some. Keep your old hard drive with all the data on it. This will speed up your system for less cost than about any other upgrade.

You say the PC slowed down. Could have many causes. Here is my list of cures.

Disk Cleanup and Defrag: Windows 7 has these tools in Accessories, System Tools folder. Use them. They get rid of unused junk files that can slow you down, and make files load faster by being contiguous. Use a good virus checker, since viruses can really slow down your machine (and steal your private info).

VIRUS CHECKER/S: If you have a virus checker, these have overhead since they really do look at all network activity. Some people install two virus checkers, which fight each other and kill performance, so don't do that. Pick one. I suggest Avira Free Anti-Virus, it's almost free and works great, with a few browser adds being the price.

CLOUD SERVICES: If you started using the cloud for backup, that can slow you down. Every file change is echoed to a server somewhere on the net, and that's a lot of waiting.

BLOATWARE: Bloatware, that is extra programs that got installed when you installed some free program, can drag you down. They watch all your network activity and share it with advertisers, which wastes your machine time. Bloatware is not free, it taxes your performance greatly. Uninstall any programs you don't use, or don't recognize.

BROWSER ADD-ONS: Check your browser add-ons. Some of these do needless update checks all the time, so disable any you don't need.

STARTUP PROGRAMS: Check your Startup file. Again, disable what you don't want.
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REGISTRY CLEANUP: Over time, a critical resource, your System Registry, gets bloated with fragments, unused entries and other garbage. Windows reads the Registry all the time, so don't make it waste time reading junk. Use a Registry Cleaner. Be very careful if you install a free one, decline any optional programs and offers. Bloatware, again.

One Registry Cleaner I like is Avira System Speedup. This is free for one single use, all you need for now. It comes with Avira anti-virus, yes, bloatware, but in this case useful. Run it once and it removes unused registry entries, unused files and fragments, and just generally cleans up garbage that may be slowing you down.