Sore hands and arms and shoulders may also be a symptom of having a tight "death grip" on the bars. Ideally, you want your hands and arms to be a bit relaxed so that every imperfection in the road isn't transmitted directly to your hands. If you're prone to the death grip, you may find that different grips or handlebar tape help. Look for something with a bit of gel or padding.
That said, if you decide you want a new fork you need to know the distance from the center of your axle to the crown race. Ideally, the numbers for your replacement fork should be similar to the stock fork. If not, you may find the handling of the bike changes dramatically... and not necessarily in a good way. The good news is that many "mountain" bikes with rigid forks are designed so that the fork can be replaced with a suspension fork that has 80 or 100mm of travel. If you're riding off-road trails, this is a reasonable upgrade to consider. If you're riding on the road, you probably want to find a different solution.