I'm guessing you do quite a bit of gaming.
So more RAM will probably be helpful but not necessary.
Going for SLI can be a problem -
You must check that your Motherboard and PSU can support a SLI configuration.
If so, then you need to know some things - you can't mix and match Ti models with non-Ti models, if one clock speed on a GFX card is higher than the other then the higher one will become the same speed as the lower one if that makes sense, they need to be the same chipset and have the same amount of ram.
I'm telling you this just in case you don't know it already.
Hope it helps, Chris.