Hi there,
The iPod touch/iPhone OS is about 240MB at the moment (has been as high as 245MB but it's been refined for 2.1) So your 8GB will be less 240MB.....but you have to take into account the amount of space 'lost' when you fomat any drive.
I have an 8GB touch and it's capacity is 7.1GB once it's formatted and the OS is on it.
I have 678 songs, a 2 hr movie and numerous music videos on it together with lots of apps and still have 2.3GB left so don't worry!
Hope this helps.