What resolution do you play games at? At 19" (widescreen or standard, doesn't matter) your 8800 GT should be a pretty good contender in most games at the native resolution.
If you have a bigger monitor, I'd say the 4870 1GB is a great upgrade. I have the 512MB version now on a 24" 1920x1200 monitor, and I can max out most games. The extra VRAM would definitely help with the higher resolution and increased levels of AA/AF as well as allow for higher settings in GTA IV (stupid VRAM limits...).