Do i need to see the original?

I would recommend that you watch them in the exact order that you posted. You will get the most out of SAC that way, but be prepared to get confused. It wasn't until my second run through that I began to "get" all of it. I would also recommend that you read the first manga series, but that may not be your cup of tea.

As you said, the stories are separate. The TV series takes place in an alternate time-line from the films and manga, kind of like a "what if the events of the films never happened?" sort of thing. Because of this it is certainly not necessary to watch the films in order to enjoy the TV series, but here are a few reasons why I think you should:

1: The films are excellent, and they help to clarify the setting and main characters a bit. The first film contains what is probably the most straightforward storyline of the entire GITS universe, but it is in now way simple. It helps to prepare you for what you're getting into here, and it eases you into the complex narratives that follow.

2: The TV series contains all sorts of homages to the first film and manga series, including the revisiting of themes, story lines, ancillary characters, and even whole scenes lifted directly from the film and re-purposed to fit into the SAC time line. The first episode of the 2nd GIG has many of the same elements as the first scene of the film. This holds true even more for the manga, and in fact the final scene from the 2nd GIG is lifted directly from the beginning of the manga. You won't catch any of this unless you have seen the films first.

3: The philosophical themes that are introduced in the film are further explored in the TV series. Things like "What is intelligence, and how is artificial intelligence different from natural intelligence?", and "What is the ghost in the shell, and can it exist separately from the physical body?". These themes really get at the heart of the GITS universe. The films begin to explore them, but the TV series goes into much more detail.

4: The first film really helps you to understand the Major better. The character of Motoko Kusanagi is (purposefully) very hard to wrap your head around. She is easily the most complex character in the GITS universe. She is full of conflicting emotions, but she rarely lets those emotions be seen by others. In the first film she lets her guard down to the Puppetmaster, and reveals her true character and motivations. She does not do this again until she meets Kuze in the 2nd GIG, so the first film will help you to understand what drives Motoko to do the things that she does throughout the TV series.

Regarding the dub versus sub question, it really comes down to preference. The subbed versions are certainly more accurate translations from the original Japanese, but for most English speakers the dubbed versions are more enjoyable to watch. The English dubs are very good, and are on par with Cowboy Bebop and Samurai Champloo. A few caveats though: I don't think that a dub exists for the 2.0 version of the first film (I could be wrong), and the second film was only dubbed for the European release. That may be hard to find, but Google is your friend. Personally, I prefer the dubbed versions.

One final thing. If you enjoy comics and hard sci-fi, i would highly recommend the 3 manga series. They are excellent and will help you to see the GITS universe for what it really is - a fully believable near future cyberpunk setting with memorable characters and deep philosophical undertones. The very first frame of the original manga depicts a microprocessor with living neurons growing into it. It really gives you something to think about in regards to recent developments in these exact types of experiments. Are real life cyberbrains that far off? After all, the Net is already a vast and infinite place...

Wow, that's a lot of words, but I hope you read it and it helps.