The auto-detect in this game is next to useless. Requires manual tinkering to find the optimal settings.
You have good RAM. Your CPU could bottleneck overall performance. I'm not too sure on the card, but it is a lower 400 series, so I am leaning towards it also underperforming.
The Witcher 2 is a very demanding game.
Make sure to keep your shadows low, keep ubersampling off, and all blur setting. Blur just games the game blurry, ubersampling nom noms your performance like a fat kid would cake. --- Shephard. Wrex. Shephard.
Dunno how much of a problem the CPU is, I run it at high and then some other extras with Athlon x4 @ 3,5 ghz. The graphics card is a hd5870, so I'm guessing that his card is the weak spot. --- Athlon II x4 (Unlock x3 450)@ 3,52 ghz, Scythe Katana 3, Powercolor HD5870 PCS+, 4 Gb Kingston 1333mhz DDR3. ASUS M4A88T-M, V7-Chassis. So small, so powerful.