1) No
2) No
3) Hm not really, if you get detected at 20 to 40m, you are pretty dead no matter what you do!
4) Nvidia card here - sorry. but make sure your drivers are upto date at
http://ati.amd.com/
5) You will have about twice as much game content as you do now. Play with with GWX for a week and you will wonder how you ever played SH3 without it.

(Im not going in to any detail as this forum is already rich with info on GWX!)
6) The only bad thing about it -is the long load times - (but its worth the wait), you need another tool called SH3 comander which you can use with GWX, You will love it, this tool will allow you to set the water in GWX to be as clear as you like - but it also does a great deal more than that!.

Download it here:
http://www.users.on.net/~jscones/software/products.html