How many gb will updating my iPod Touch's software take up?

I want to update to iOS 4.2x, but I'm kind of scared to do it. I have updated my iPod Touch once before, to iOS 4.0x, but that took up a lot of memory. Lately, it seems like my iPod Touch's (its not the newest one, with the camera and everything, its older) memory has been going steadily down. When I got it, it was 7.8gb or something (it was supposed to be 8gb), but now it is down to 6.83. This is ridiculous! My question is, will updating to iOS 4.2x, and after that iOS 4.3x, eat up even more memory? And if it will, how much, exactly? Thank you!!!!!!!