ronval - This is my understanding of it, there could be misinformation.
If you use the low quality scaling, it will decrease GPU load and increase overall performance to a comparable level to if you were using all the assets at that lower resolution, even if the assets have extra pixel information in the editor layout. But you will still have the RAM commitments for the original size of those assets.
For example, if your layout is 320x480, you can still have assets in your game that are 512x512, bigger than the layout itself. These assets with higher resolutions see use when using the hi quality full screen scaling, but use the same RAM regardless of render mode.
When you use hi quality scaling, it takes your 320x480 layout and scales it up to fit and render at a resolution of the device. So if you are running it on an iPad it will render at 768x1024 and on a 3GS it still renders at 320x480 because that's the native screen size. So while 512x512 is overkill for an asset on 320x480, that extra pixel information is used when scaled to fullscreen in hi quality.
Since the aspect ratio of the game is the same scaling from 320x480 to 768x1024, what IS changing along with the resolution is the ppi (pixels per inch) or how close together the pixels are. Retina displays have very high DPI displays, meaning that even though the iPhone 5S isn't much larger of a phone than the iPhone 3GS(163ppi), it has a much higher resolution (1136 x 640 pixel (326ppi), higher than the iPad2 (132ppi)!) so a game rendered at 320x480 will look great on 3GS, and pixelated on the iPhone 5S. That said, the 3GS looks more pixelated overall than the iPhone 5S due to lower ppi, because the pixels are larger.
However including higher resolution assets comes at the cost of RAM; each 256x256 square of pixels you have uses 0.25MB of RAM when loaded in the game. Meaning a 512x512 square uses 1MB by itself! This is what vtrix means when you should do both to save texture space, to modify the resolution of the original assets in the game to more closely match the intended resolution (ONLY IF USING LO QUALITY AND NOT GOING BACK) so you could potentially achieve the same effect with a 64x64 px sprite on lo quality as a 512x512 px sprite.
If you run lo quality, your game always renders at 320x480 resolution and then scales those pixels existing at the 320x480 resolution to fit the device's resolution of 768x1024. So you aren't filling each pixel with it's own unique color information, you are kind of stretching one pixel's color into like 9 pixels of information, causing it to look artifacted and pixelated.
I kind of got off a few tangents here, so if anything is unclear let me know and I'll try to explain further.