"Dual card setups don't give big enough performance boosts (over a single card) to start talking about quad card setups"
The thing with two cards in SLI is that the MIN frame rate is so much better. I use two 20 pipeline 7800GT's to hold down the silly cost of 24 pipe 7800 models and spend the savings on 2GB of RAM so load times are much lower. Why does that matter? I suck at these games and keep dying and have to reload all the time. Laggy min frame rates seem to help the AI kill me better than I can kill it, too. I use standard PC3200 Corsair value RAM locked to 208MHz when I overclock the 3800+ X2 to 2.225GHz. This gives you 4200+ performance, two SLI cards and memory that is 1/3 to 1/2 as expensive compared to what most people buy with and with single card! I just don't see the min frame rate performance advantage going the route of fast memory and a fast CPU with one video card.
This wasn't a terribly expensive PC box (no monitor, Mouse or keyboard) if you watch what you buy. A sensible CPU and RAM leaves the extra dough for two SLI cards instead of a less effective purchase of super fast CPU's and way faster memory than what really helps. There are even sub $100.00 motherboards that run SLI very well (ASRock has a $85.00 model that rate well) and could let you still buy XMS style memory for an OK overall BOX price.
Also, you get a PC that works well right off the bat verses, for example, a XT800XL that barely runs current games, only to upgrade to a single GPU later on that barely runs the next engine game and to NEVER get to run ANY current games extremely well today. You can wait a year or more till the new games are old and the hardware to run them is finally cheap. Not a bad option, either. Me, I like to watch old movies when the are two bucks and play semi-current games. Pick your poison.
So, two video card are NOT necessarilly too expensive or headonistic, and if done right, down right practical, too.