227: "1.73 Ghz"
Not exactly well-versed in computer parts, but I'd say that's your problem right there.
tahirahmed: Generally speaking, laptops are not supported for games. Heavy games push your hardware thus making them hot and since laptops are designed for portability they don't have good ways of cooling the hardware besides as 227 mentioned 1.73 Ghz is not good for a game as heavy as this and it is a bottleneck for the rest of your system.
Things you can do regardless:
1. Have you updated your DirectX ?
2. Your mobility radeon 5850 is a good card, if you haven't updated your drivers yet get Catalyst 11.5 because I noticed a huge reduction in choppy and jerky gameplay when I updated my drivers from catalyst 11.2 to 11.5 so it might give you some extra fps.
3. Try to use your laptop near a cool well ventilated area because once your hardware run hot things start getting worse, you loose fps, hdd takes more time to load (thus choppy gameplay) and more chances of crashes.
4. Disable extra programs, specially antivirus programs if you have any.
5. Consider getting a gaming PC, my Radeon 4870 which is quiet old with a Q9550 2.73 Ghz and 4 GB ram is outputting much better performance than 20 fps and with everything turned on high (only ubersampling off).
Good luck.