In the movie, after his character loses his legs, the filmmakers used a combination of standard camera 'tricks' and digital effects to give the illusion of a legless sarge. For certain scenes, the actor wore blue socks similar to a 'blue screen' (or green screen nowadays) used in the back ground when doing a digital effects shot. Like a blue screen, the blue part was digitally erased in post-production and replaced with another image, mainly the tops of the stumps of his legs and the background under his legs to make it look like they weren't there.
The special effects team behind Forrest Gump was George Lucas' ILM - Industrial Light and Magic. They've been behind most of the breakthroughs in special effects since they were founded and Gump was no exception. They used new technology (at the time) to blend Forrest seamlessly into the many archival news shots in the movie, and even to digitally create the feather that floats down at the start of the movie. d
I usually only answer one question per person per week so I'll leave your 'casualities' question for now. This also is kind of a subjective question- it depends on your criteria for 'casuality'. Would it be people you actually see get killed, or just people who die in the movie? For example, in the animated flick "Titan A.E", Earth blows up so there would are a few billion casualities, but of course you don't see each death. .
If you mean a computer being the main character in a movie, not a CG-character, that is a little more tricky. The earliest I can think of a computer being an actual 'main character', and not just in the movie, would be 1968's "2001: A Space Odyssey". The earliest full-length major movie I could find on IMDB that has computers as a central part of the story would be 1957's "Desk Set" - here's the link: http://us.imdb.com/Title?0050307 However, the computer is not a 'character' per se. .
Both movies were controversial - the original was extremely racist; it's hero was a KKK member and all blacks were portrayed as either ignorant, violent, or both. The sequel was basically a propaganda piece for WWI to get Americans involved in the war- Germany invades and seizes control of America in the movie. This was the earliest true movie sequel I could find noted anywhere- I'm sure there were little one or two reel sequels playing to short films in the coin-operated movie houses, but these were not 'movies' in the terms we think of them nowadays.
As a side note, the first movie sequel to feature a '2' in the title, was the 1957 sci-fi flick "Quatermass 2". The sequel to the 1955 genre-fave centered on a scientist who uncovers an alien plot to take over Earth.