New PC ($300-600$)
If you feel that upgrading may be too costly or you need a new gaming PC, then this is the choice for you! It is too the point, no hidden fees (maybe tax), and you get everything in one peace without having to open a PC.
Ram upgrade ($30-$100)
If your PC is less than 3 years old, and is just a bit slow, this could prove wonders for you. But just always open your PC and check the motherboard model number before you buy the ram, this tells you what type of ram you need, and how much ram your PC can actually use. If more ram than the maximum amount is inserted, then all of it won't and can't be used!
Hard drive upgrade ($150-$500)
Hard drive prices vary greatly since they come in a number of sizes and speeds, I would only recommend this upgrade if you are truly running out of space, try looking for other alternatives rather than this like an external hard drive, or multiple flash drives. The reason being is that this upgrade can be costly, and it does involve opening the PC and a bit of work.
Graphics card upgrade ($30-$300)
This has a huge price range, and I only recommend this to you gamers or photography/video related people. Since this is costly, it would be cost effective to buy a new PC, and some graphics cards may not even be supported by your motherboard. And a few cheap $30-$100 are not nearly as good as the more expensive ones and will rarely feel like an upgrade to your existing graphics card.