Finally today I have been able to build my new computer from all pieces that were laying around from various sources. I went for Tom's Hardware recommended low-medium system and came up with following:
Intel Dual Core E6175 (2.66 GHZ dual core) FSB 1333 MHZ
Motherboard ASUS P5K
2GB Corsair DDR2 800 Memory
Nvidia Geforce 8800 GTS 640 MB (I spiced it up a bit here...the 320 MB version was the recommended one but since I'm going for a 24 inch LCD in Xmas I wanted to be ready)
The end result: SH4 1.3 stock runs with all graphic specs maxed out and high quality settings in the NVidia control pannel at an average 60-70 fps near land and other ships

, 80-100 fps when alone in the sea and 150 fps peaks in the interior of the sub
The wait for upgrading since SH4 came out has been justified I would say: Got the whole thing for around 950 €
Hope this specs help someone looking for an upgrade, though note that the Video Card is always the most important part and what counts more for game performance.