The power thing can be confusing.
The spec says (simplifying a bit) that each socket may be asked for up to 500mA. If you use a non-powered USB hub connected to a single USB port on your computer, you can easily exceed that, with potentially interesting but unpleasing results.
A powered USB hub should resolve that.
USB hard drives are mostly either small (and can live with 500mA), or have their own power supply. A few are "oddball" - I've got one here that connects to two USB ports: it needs 800mA.
With PCI cards, the general rule is that the PC is "backward compatible" - so if you get one of an earlier vintage than your PC, it should work. If you put a later one in an earlier PC, it may not.
You may still get away with it - I do, most times.