Video memory.

I already got a EVGA GTX 285 which is already a gig of dedicated video memory. Would it be worthwhile to dedicate another though? Currently running with 6g atm. Any downsides atall?

If you do SLI, you'll still only have a total of 1GB of VRAM.

1GB of VRAM is basically all you really need these days for resolutions of 1920x1200 and under.

Now for anything above that, that is when your going to start looking for SLI setups with multiple cards of 1GB or even the GTX 285s that have 2GB of VRAM per core.

I do use 1900x1200. So I should just keep it like it is basically?

Unless you want more FPS in your games, then I really see no need to at this point in time especially since newer generation videos should becoming within the next few months.