belfasteddie

If you started with one stick of 256 and then added another 256 you would see a big jump in speed. If you remove a stick of 256 and add a stick of 512 you won't see that much difference.
To question 2: You can add a stick of 512 and leave the other 256 in place.
If you want the comp to go faster then click on Start,control panel, system, Advanced,settings and click on Adjust for best performance..OK and back out.
Next, Reboot and go into the Bios and adjust your Video ram to 8,16 or 32 megs , this amount is a matter of trial and error. back out and reboot.
Next right click on a blank part of the desktop and select Properties,setttings and in the box with Color quality select 16 bit, Ok and reboot.