There are far better options that getting 3 of those cards. Such as one 4870x2, or maybe a couple of GTX 280's.
Yeah, I wouldn't splash out on 3 of 'em. To be honest, I wouldn't buy one of them. Your SLI set-up gives pretty much the same performance as one of them would.
I'd keep with what you have now, till you notice it struggling. Which will be a pretty long time.
Then, by that time, nVidia will have new cards out.
I think you're going to struggle to find somebody that's worth upgrading to from the Q6600, especially since he has it at 3Ghz. It's nothing that a little OC'ing couldn't fix.
His SLI'd 8800's are perfectly fine for now. There's no reason to upgrade. You're not noticing any lag are you?
get a pair of 4870x2's if your going to go multi, isn't much point in tri sli unless you play on a moniter with 2560x1600 or whatever it is