My dad would like to upgrade the memory on his Dell Inspirion computer which is approximately 4 years old. Once probing him a little further, he also wanted to upgrade his CD burner to a DVD burner. Well, then I set out on a quest to find out the cost benefits of upgrading these specific components of this unit vs. purchasing a new computer that would have not only what he wanted in a upgrade, but much, much more. Then I had an “ah-ha” moment. This is why the technology industry has lifelong customers. Computers and other electronics will always be upgraded, offering customers something bigger and better.
So when researching how much each component was to upgrade, I found that I am really comparing apples to oranges. Yes, it might be beneficial to upgrade the former PC to contain the specs that you want, but then again, you could purchase a new machine that has a multitude of upgrades. One major upgrade would be that of a internal wireless internet connection. Technology has changed so drastically in the last 4 years, that it is hard to recommend upgrading these specific components when comparing what is available now on a PC.
Furthermore, I might even recommend that my father purchase a Mac. Since Windows has upgraded to Vista, I have heard of nothing but problems from the consumers regarding the Vista system. Although a Mac would cause my 60 year old father to learn how to navigate on a new platform, he has a wonderful daughter who will soon have her masters degree in ILT to teach him about the benefits of using a Mac computer 
After our coursework regarding software and hardware, I think I would be able to help my father to build a system that would be current with the capability of being upgraded further in the future, when something bigger and better comes along.
You’re definitely right about computers and technology constantly being improved. There are typically multiple hardware releases per year that are implemented into new systems put out by computer manufacturers. Of course the issue is always $$. A new system often is much more than a simple upgrade. The decision comes in once you know what your dad needs to ‘do’ w/ his system. If he needs a wireless card, a DVD burner and memory.. likely yes that money might be better spent towards a newer system. The Mac issue would be another factor, but lucky for him he has a ‘wonderful daughter’ who’s going to be his tech support!