It wouldn't slow the system down, just affect resolution and your eyesight. If Linux detected a monitor that could support 800x600 at 60 Hz 8 bit color it would not allow you to set your X above that, hurting your eyes, but not the speed of the machine. You need roughly 2K fps to run tuxracer smooth at 1024x768 @16 bit color. You aren't going to get that with the stock driver.
run glxgears, report what your fps is. At a guess, 300 fps is what you will get.
Note: with the stock nv driver on my GeForce 4, I got 277 fps freshly installed. Installed the nvidia driver, tweaked a little, back up to 7k fps with glxgears. The optimized driver makes all the difference.
The 650 family appears to have a Linux driver.