First off, the bastards who sold me the motherboard sold me a GOOD damned motherboard. Replacing the motherboard with a better 4-year-old MB is a not costs effective.
I'm using an Athlon. Mine is a Socket A 2600+. They stopped making that line at 3200+. So I could spend $50 bucks to get a small boost, but nothing major.
I could add more RAM, but that doesn't add to speed. It is also an odd RAM, so that would run $65. I may do that, as more RAM is rarely bad, but I won't get much out of it.
I looked at buying a used PC. All that does is trade one old PC for another.
I could try to upgrade piecemeal, but I want a small form-factor. The house is small. Space is valuable. Think "shuttle sized" computer. mATX. I to replace the current PATA DVD burner with a SATA dual-layer. I want to upgrade the size of the hard drive. I want to replace the mouse with a more hand-friendly mouse.
At this point, it looks like all-or-nothing replacement, or waiting for the used PC's to significantly exceed the specs on my current PC.
The only thing I want to keep is my IBM keyboard. I can't find them anymore.
I specced out the price list. $600. Not this year, I think. I'm still paying for the laptop.