The size of the screen is irrelevant. What matters is the resolution. That size(28") monitor is likely to be 1920x1200. If it is a 1080P TV, then it will be a bit less.
More ram on the card has already been factored into the performance ratings. I would not worry too much about that.
SLI has been a poor upgrade path in the past. It should be used only by
those who will not currently be satisfied by the fastest available single vga card
which is currently the 4870X2. The more affordable 4870 or GTX260 cards are close in performance, and represent a good value at the high end.
Paired with a decent cpu, you will get a good gaming experience.
To get SLI. you have to spend more up front for a SLI capable mobo, a
more powerful SLI capable PSU,a better cooling case, and a stronger cpu to drive them. Upgrading a single card later with a
second equal card does not get you 2x increase, it is more like 1.5x to 1.7x depending on the game.
At that time, you will still be paying top dollar for a card that is closer to
being obsolete. It would be better to sell the old card and use the proceeds
towards a better new generation single card.