Well you seem to have a good enough computer aside from the video card.
Have you tried closing any unecessary programs that are running in the backgound or in the taskbar?
If programs are hoggin your ram and cpu power, that can cause major problems running games.
If your using XP:
press CTRL+ALT+DEL at the same time to open the task manager.
Go through the Processes and close the ones XP lets you close.
If your using 98 or lower:
press CTRL+ALT+DEL at the same time to open the close program box.
Close all programs except for Explorer and Systray.
Close the task manager/close program box, run steam and play.
If you see no improvements, try different video card drivers. Try uninstalling some programs off your comp. If your hard-drive has a lot on it, it takes longer to find the sounds and stuff for CZ causing a drop in performance.