I was actually interested in how big an effect on performance would decreasing the resolution would be. And what the effects would be on what I see: e.g. does the interface get screwed up?
The performance difference will depend on the game, your system, and the resolutions. The only way to know how it will affect you is to try it and see.
The interface
shouldn't get screwed up (though it may be larger) if the game supports the new resolution because any resolution option within the game should have been tested, of course not every game is well made and well tested. However, as with any aspect of computer games, you specific mix of hardware and software could create a unique issue at random resolutions. This is unlikely but it can happen.
If you are forcing a resolution not supported by the game, it may or may not screw up the UI (or the entire game) and the only way to know is to try it and see. Otherwise, the biggest risk to the UI is that you select an aspect ratio different from your screen and strethc everything horizontal or vertically.
For other aspects, it is just the opposite of above, you could just have everything bigger and show less on the screen and/or everything is the same size but with less texture detail.
Really, when playing with settings to affect performance the only thing you can reliably do is try it and see.