I was trying figure out the minute (or not so minute) differences between various hit point and damage systems. For that I just made a simple spreadsheet (available here) where I calculated a number of things - I thought it may be some help to others.
I provided a minimal amount of notes (how I obtained the average HP and damage at a given level), but the following should be notes, as well. The first and sheet includes how many HP a character (or creature) may have at a given level and how much damage he may deal. Assuming an opponent of equal level or Hit Dice, you can find the number of hits it may take on average; after that, how many attacking attempts are required depending on your chance to hit (25%, 50%, and 75% of success, respectively).
The third and fourth sheets differ: they show you how a character of a pre-defined level (1st, 3rd, 5th, and 7th) compares to creatures of various Hit Dice. Therefore, average HP is calculated for the creature and damage for the character, and the number of hits or attacks are calculated from the character's point of view.
2014. 03. 13. edit: Last night I updated the file for there were some incorrect functions. Shouldn't have done it so late, I guess.