I agree with Stabiz on the hardware issue, mate! He knows what he's talking about! (Check out his Excellent movies, btw!!)

The screenshots: I use Fraps or simply Ctrl + F11. The images are stored in my SHIII folder (dont know why with Ctrl + F11, but for Fraps i have directed them there.)
I dont remember what my best sortie was, because i have played SHIII so long that i cant even remember my first patrol. But i dont think it have been over 60000 tons.
Edit: best tip: Take your time, dont rush anything. Learn the DD's movements around the convoy, watch out for sudden course changes. Try to forsee where the convoy is heading, run silent & deep, and try to get to periscope depth after the first escort have passed you. If you get inside the convoy, you can do alot of picking before the escorts get a hold of you. But beware for those armed merchants..
Hope this helps a bit, mate!