The "going well" part makes me want to advise, just let it run, Harry.
If you are able to perform, with no problems, the day-to-day tasks for which you use the computer, be it Internet use, word processing, watching videos, balancing your checkbook, listening to music, or "making/burning" discs, or whatever, then you're fine.
The first area in which you'll probably notice the computer's "age" will be, I'm guessing, the Internet, as the 'net continues to become more and more graphics-intensive. (I've no specific time frame in mind.)
If you were a big "gamer," the intensity of gaming graphics demand more and more processing power from the computer.
The advantage of waiting to buy a new system is that you continue to get more value for your pound, as processing power increases.
Any "folding" contribution is significant and welcome. If you arrive at a conscious decision on your part, for example, to process more "folding" work units, then you'll need a better system. As I mentioned in another post, your current system is quite respectable.
The other factor, down the line, is parts failure. A CD-ROM or DVD drive replacement is fairly inexpensive on a desktop computer, and no cause to replace the entire system. Same with a hard drive. A motherboard failure gets you to the point where you might decide on a whole new system...or if enough individual components fail at the same time.
(I once had a laptop's video, hard drive and CD drive all quit around the same time. The replacement / repair of any two might have been worth it, but all three going -- less expensive to buy a new laptop.)
