Anvar1061 advice is sound for someone with a weak PC, but looking for a realistic and tough campaign. You will not go wrong with NYGM. I would also recommend using H.ies's patch and Striebler's 4C patch with NYGM. Neither should be hard on your PC, but add quite a bit to NYGM. Last and not least SH3 Commander is well worth using.
Read about SH3 Commander here:
http://www.subsim.com/radioroom/show...t=our+mod+list
H.ies's and Striebler's patches take a bit of careful manual work but really are not that difficult to install.
Good Luck
Kelly621