Majik - I've heard the 500 & 600 first-hand (not side by side unfortunately). In both cases, the subs were in medium-small rooms ~(14x10x9). The differences between the two are fairly subtle as Jay mentioned, but they are both very impressive subs. When I ordered my set, for me, the little bit extra was worth it for the 600 versus the 500, as I'm in the same camp Jay mentioned "you can never have too much sub."
I was then bitten by the same bug again when the 800 came out, even though I'm extremely happy with my 600 now. That's a long way of saying that assuming you have a medium-sized or smaller room, you'll be very happy with the 500. The differences from 500 to 600 and 600 to 800 (I suspect) will be more subtle, but definitely there. It comes down to a personal decision of price to value.
On the question of 1 versus 2, I'd definitely just get the one first and make sure you 'need' another one before you put the money down. You might find that you're very happy with just the one sub.
Jason
