The "90w" does not describe how much power the adapter puts out, but how much it's CAPABLE of putting out. So if your laptop requires 65w, then anything 65w or larger will be fine, as long as the voltage is the same, and the plug fits and is the right polarity.
So: The 90w adapter from Dell that is intended for your laptop plus a docking station will be fine, even if you're not using the docking station. If you use the docking station, you should use the 90w adapter rather than the 65w adapter, or you'll get a warning from your PC that you're using the wrong adapter. You can turn this warning off in the BIOS. Nothing will be damaged, but your laptop will likely not run at full capacity (perhaps the screen will be dimmer than it ought to be) if you use the 65w adapter with the docking station.
There's nothing "special" about the Dell adapters, except that you KNOW the plug will be the right shape for the machine, etc. Using an off-brand won't hurt anything, as long as you make sure that the polarity is the same on the plug, and the outputs (voltage, in particular) are the same.
