Hi Dirk,
I wholeheartedly recommend several plugins from NikMultimedia.com. DFINE, CEP Filter Set, and Sharpener Pro. I would suggest you go to their website where you can get detailed information, and perhaps download the user manuals, on these products. The Filter Set comes in three "sizes", depending on the number of filters you want. These are not just static type filters. On graduated filters, for ex&le, in addition to controling the opacity, you can position the transition line anywhere up, down, or at any angle. They are really great. I personally find DFINE an essential part of my tools, as is their sharpener plugin. The only downside of NikMultimedia's plugins is they are not cheap.
Fred Miranda also offers several excellent plugins that are very reasonable. FredMiranda.com
And lastly Extensis has a two excellent plugins, one called intellihance and the other is called PixlScale which lets you blow files up significantly with little loss.
Some of these plugins overlap in functionality, so its a matter of taste and price. I do believe that a sharpener plugin is essential though as they allow selective sharpening (application of Unsharp Mask) to various parts of the image in varying strength.
My favorites are the NikMultimedia ones, even though they are pricy. IMHO, they are tremendously flexible, have superb user interfaces, and excellent documention. But I would look at all of them.
Good Luck. If you want I can send you copies of the manuals for any of the above as I have tried them all.
Gary