At a certain point it just makes more sense to move on to a newer version of Windows, as Windows 7 is no longer supported for security updates. I see no reason to just stay at windows 7 now, unless you have a program that absolutely won't run in Windows 10 and you don't want to find a better program. Ryu is right, a SSD may surprise you on how much quicker your system will be on startup and running, but adding memory will also help. If you go new, you probably can triple your system performance though, and future proof a bit.
I would run UserBenchmark.com to see where your system ranks. I just ran mine, and got a "Battle cruiser," as a desktop, fine for what I do, but I don't do gaming stuff or need hi-res graphics.