Although it MAY require more memory, the GPU is specifically optimised (in MOST cases, as I said above) for fullscreen applications, and thus, even though it uses more memory, it actually runs considerably faster.
Also, that shader you're using, I can assume, would prefer to be used fullscreen over a window.
Just remember, you CAN lower the resolution and add the option to up it again... should the performance demands not be
too high. A lower resolution also means less work for that shader (and any framebuffers, should you choose to add them... IE: motion blur, CoD4 style explosive shockwaves, etc... in other words, the cool looking eye-candy

).
Now, all that being said... remember my saying from an earlier post??? "Every cycle is infinately valuable" or words to that effect... your game IS going great, but there is ALWAYS room to optimise
more. Right up until the minute that you release this, I really want you to use ANY optimisation method that is humanly possible. I'm really beginning to look forward to this, and I honestly don't want to see a thread full of people (with low-spec PC's) complaining of lag. It's too good for that. It's above it. And I'm personally willing to assist in ANY method possible to help you optimise it, so that it gets the respect it rightfully deserves! I know that one person can only do SO MUCH before becoming "weighed down", and I want to help you "lighten the load".