Why did you order two cards? The 8500GT will work perfectly fine with no hassle although unless you were looking for the HDMI port you should have gone with the 8600GT for better performance. Also the 9600GT isn't necessary easy to install unless you have a PCI-e power adapter coming out of your PSU which I have never known a Slimline to have.
Also if your screen resolution isn't right, see if your monitor has some settings to correct the issue. It may not necessary be the video card's fault. If your monitor came with a CD that has some display drivers on it then you do need to install those too.
Also if your screen resolution isn't right, see if your monitor has some settings to correct the issue. It may not necessary be the video card's fault. If your monitor came with a CD that has some display drivers on it then you do need to install those too.