hail Mihajlo. despite all the bad press i'd seen/read about SH4 i decided to give it a go about 2 months ago. it wasn't nearly as bad as i'd expected, and i gotta say i play it far more than SH3 now. go for it! you'll enjoy i'm sure
as for specs, i currently run:
AMD64 X2 2.2 (dual-core)
nVidia 7300LE 512MB
1 gig RAM
it runs perfectly well with this setup, and doesn't even really slow-down much with a big Task Force/Convoy on screen. in fact, the only time it runs slow is when i bring up the Radio Messages during a long patrol, when it struggles to cope with all the received messages (most of which are pointless anyway). this is prob down to having only 1 gig memory though.
hope this helps