Aces High is CPU-bound, not video card bound, so getting an SLI-like setup is a total waste for this game.
For other games, I think the math in favor of an SLI-like setup is very poor.
You get to spend TWICE as much money, use TWICE as much electricity (needing a more powerful and expensive power supply), make ALMOST TWICE as much noise with two fans spinning like crazy for both cards (plus any extra system fans you need to keep the system cool overall), all to get between a 20% and 50% increase in framerates (usually closer to 20%) than you would with a single card.
I think it's much smarter to spend $250 to $350 on a good single card than $400 ($200 X 2) on an SLI-like setup. And for AH, $150 will do for a good card.
-Llama