With an Apple Computer, you are dealing with one vendor. I never owned an Apple, but I am not certain you can change out video cards or sound cards or add extra memory.
Even if you can, you are going to be limited in your choices.
There are plenty of programs to use on an Apple, but not as many as a PC.
Apple are good computers. They cost more and differences in the operating system has never been enough for me to change. I am using a Dell Percision that is about 10 years old. I am running Windows 7. It has eight megs of ram and a video card that supports dual monitors. I do want to change my hard drive, because it and the power supply are the two parts most likely to fail. Changing a power supply is easy. Cloning your existing hard drive to another disk and then replacing it in the machine is a little trickier, but doable.
The one thing in Apple's favor is that when you buy a new one, you just turn it own and it starts. A new PC requires a series of Set Up steps.