Kwin comes with it’s own effects as well, hell it even has the famous cube.
Since you only have 64MB of videocard RAM I’m guessing the problem lies there.
To disable the effects completely, go to the KDE4 menu and search for “Configure Desktop” (by default already on favorites though… first thing you’ll see).
Then go to Desktop, deselect the Enable desktop effects checkbox and apply the changes.
You might instead of completely disabling setting everything to “no effect” and leave window management on… since it’s quite nifty and I doubt it’s actually related to anything the videocard is used for.