My wife was out of town yesterday - a rare occurrence, we are seldom apart - and of course I got up to some nonsense. Pretty benign nonsense, but somewhat indulgent nevertheless.
Our main home PC is a home-built affair, it's several years old and also quite unremarkable. I previously upgraded the hard drive to an SSD (a great upgrade, I have to say), and that's been about it.
Anyway, it was built with an Intel dual core processor because that was available and cheap at the time, although the motherboard actually supports a quad core cpu.
I recently found a suitable quad core (an Intel Q8400) on Amazon for $15, so of course I figured it made sense to purchase it at that price. I also realized that the CPU fan (which I had previously cleaned a few months ago with compressed air) was clogged again, so I also purchased an inexpensive replacement fan ($10).
Since the wife was gone, and she is the one who primarily uses that computer, I pulled it apart on a Saturday morning, and after some sneezing through all the dust and dog dander I replaced the CPU and the cooling fan with no drama.
I was a little concerned that I might have an issue at boot time, or with Windows activation since I had changed the hardware, but everything came up, Windows updated the drivers automagically, and all was well.
It's not a huge performance boost - I had not expected it to be - but it is certainly measurable, and CPU-intensive stuff is improved. I would say I did get $25 worth of improvement, so that works in my book.