Yes, I used to crop all the time for screen use, flower shots and detail and the like. I you can crop 100% but it's the final output size and viewing distance that matter. I have 20D A3's that look good to me and a 100% crop filling the screen in CS5 gives 1364 x 735 at 998kb which at 1000 wide has to be saved at quality 11 rather than 12 and ends up at 454kb and looks like this.
These will lose some quality/sharpness for being posted here.
The whole picture has to be saved at quality 10 at 1000 pixels wide to be posted at under 500 pixels wide here.
This all depends on the intended use. A 100% crop won't make a staggeringly good A3 print if you stand with your nose against it but may well look lovely at A4 viewed anything like normally and should be adequate for a picture to be viewed on screen.
I do understand the urge to buy new kit but when anyone says things like the op has about image quality or other issues I do wonder if these same issues could be carried over when the new kit arrives and comes out of the box. IMO it's maybe best to look at the causes of issues before moving on.