I want to know about XSI, should I know it befor I get in to Zbrush or should I know zbrush befor I tackle XSI? Or do I even need XSI?
xsi is better to know first, but im so drunk that i dont know what im saing :] happy new year, im going back to party with the girls 
It depends… what do you want to use ZBrush for? Some people just use ZBrush for still images and as a result do all their work within ZBrush. Others just use ZBrush for detailing and bring their model back into their 3d animation program for rendering.
Sometimes, even though they do a finsihed piece within ZBrush, they might start off a model in another modelling program and import it.
think of xsi or any other 3d package like a the main program. Or you could think of Zbrush like a main program. But thinking of zbrush as a main program would be kind of weird because you make a detailed model in zbrush and then you export it into a 3d package. Well if you want to make your model in zbrush and then export it for animatiopn and renderintg than yes you could think of zbrush as a main program. From many people that i have heard they say that first they make a basic shape ( low poly model) in their 3d package and then export it into zbrush. MY opinion is that zbrush is more of a " detailing program"
hope i make sense
small_orange_diamond
A lot of people also do their final renders in other packages such as Maya or XSI.
So it’s like another Maya?
Yup, except that Maya has Industrial Design with Alias in it’s pedigree. XSi was created with only Character design in mind. So depending what kind of work you do you shouldn’t really think of them as the same. With that said, most folks would indeed find them interchangable.
What sort of work are you looking to do
I want to go in to movies
Well you have to start somewhere.
What aspect of movies? CG enhancement or 100% CG. Do you want to be a texture and/or detailing expert…if you look at the credits for any CG movie you will see quite a lot of people involved each group specialising in a particular aspect.