Thanks everyone with the replies.
I think I'm going to go pick up Silent Hunter 4 there in the coming days and wait to see if I enjoy the series. If I do I'll pick up SH3 from this site(as some of you suggested) and enjoy that one to the fullest as well. Besides I have a tendency to play a game I enjoy pretty much non stop for awhile and I would rather get both at separate times so I'm not always splitt between them.
I know my computer has the specs to run SH4 and it should be compatible with XP, being as all specs for the game have said it is compatible with XP. Once I do pick it up I'll be playing through that vanilla(unmoded) version of it to get a feel for what it's like and then gradually as I read this forum add a few mods(maybe even ask what some essential ones are). Like I said in my first post I have been watching quite a few videos on Youtube and have been getting more and more into SH4 and if it looks as good as in the videos I'll be playing it for many months and maybe picking up SH3 in the not so distant future.
So thanks again for the replies and if anyone else has anything to share, please go right ahead.