I presume since you did not mention mods, that none are installed. If mods were installed in the past, did any mod files remain in your SH4 folder before a new install? That has caused errors in the game even if all mods have been disabled.
My machine is 2.6 quad core with 4 gigs RAM, two crossfired Radeon 4670, and Soundblaster. XtremeGamer Fatal1ty Pro Series and 600 watt PS. I play a lot of mods mainly GFO, TMO, RSRD and Oper Monsun plus some miscellaneous ones. I have had weird crashes with mod soup leftovers. I have had weird crashes with torpedo explosions, too.
The task manager sometimes says I am using 1GB to 1.5 GB of RAM for just SH4. The 1.5 GB of RAM being used at the very moment is usually when torpedo explodes on the ship and the game freezes.
Memory cleaner from
http://www.koshyjohn.com/software/memclean/ has eliminated some of my crashes. It is a much better memory performance tool than others that I have used in the past. The options section to trim processes' when usage exceeds 80% and trim processes working set every 5 minues helps the most. I usually save the games a little more frequently when I sense the game will lock up (convoys or at a point where game froze recently before). After loading to the main SH4 screen. I sometimes clear the memory process, 600-700 MB will be recovered. After a loaded game and a process memory clear, SH4 is running on 4-8MB of RAM. It will eventually increase. I usually alt-tab and cancel the 5 minute memory clear if I am within a big convoy or just about to blow up a ship sometimes.