I have invested some decent (but not stupid) money on my home stereo, but I have realized I listen to my crappy computer speakers about 3 times more.
So here is the first step in my upgrade, a pair of powered studio monitor speakers. At under $200 a pair, they are not that much more expensive than higher-end computer speakers. These are built for near-field listening and sound much better than most computer speakers. The one I chose are a tad large, but I have loved them so far, even without a subwoofer. These particular units are cheaper at Amazon by $10 but Sweetwater has better selection and help choosing studio monitors. Just make sure you have the cable situation sorted out -- most studio monitors take a special 3-prong balanced XLR input common in high end and pro audio but not in most consumer audio. But you can easily find a number of converter cables.
If I keep listening at my computer, the next improvement step is an inexpensive DAC (which can correct a lot of mess in computer digital audio).