Photoshop 5.5 isn't any good without the plug-in's. Currently I'm on ver 7.
If PSP is doing what you need it to do, just stay with it. It's easier to manage than Photoshop.
To be honest, my wife uses PSP and I use Photoshop. I'm not sure what the differences are, other than Photoshop is harder to get around in, but once you learn it, there isn't much that can't be manipulated.
On thing that stands out the is the ability to create and save your own styles.