No game 'should' be worth upgrading hardware for.. but it's definitely what motivates me to do it!
Infact my first ever PC was bought because I saw Aces of the Pacific in a computer shop and was so impressed by the screenshots and description of the game I bought it even though I didn't own a PC. The following weekend I spent £1500 on a 486DX/33 with 4 meg ram to run it on.
Sometimes a game/sim comes along and logic just goes all out the window
Whether you should upgrade for SH4? I would.. but there are plenty of people here that would probably tell you it wouldn't be worth it if you can run SH3 with mods.. best option is to decide for yourself based on reviews and forum comments if the outlay is worth it to you. You'll have to upgrade soon though, because more and more games coming out just won't run well on anything but a newer model card.