With GTAIV, it's the vram amount for sure. That game can be a huge hog of vram.
I dunno if dropping view distance in that game reduces vram usage or not, but it's worth a try. Also worth trying is using lower texture size/quality (whatever it's called), and don't use AA (does GTAIV even support AA?). The other option is to lower the resolution, which is the most effective way to cut down on vram usage.
In SLI the memory of all cards in a SLI set *is* used, however, because SLI works by alternate frame rendering (iow the cards 'take turns' rendering entire frames), the total amount of info that needs to be stored in memory gets doubled. This is because both cards need direct (i.e. on-board) access to all the information necessary to render the entire scene.
Thus, the 'effective' memory capacity for the SLI setup remains at whatever is present on a single card.
As a side note, you ever notice it takes way longer to load levels on an SLI setup than it does w/a single card? It's cause you're filling up two cards memory w/data instead of one.
Lastly ... if you run Afterburner you can fire up the onscreen display (OSD) and have it show you the vram usage while you're playing games. If you get up to around 1GB usage (the meter won't go over what you have), you're gonna take a perf hit.
Edited by brettjv - 4/14/11 at 3:01pm