Well, like I said, when I'm on the freeway, I put it in the overdrive gear. But around town, going max of 45-50, I've been keeping it in D3, and riding at about 2.5-3k.i'd say, unless you're cruising on the freeway at 4k rpm then you're putting a lot more stress compared to the overdrive gear which would probably around 2-3K at 60-70mph and wasting quite a little bit more of gas. The only thing you would have to worry about is making sure you maintain the transmission and lets say probably more tranny fluid change intervals and maybe engine too. If gas isn't a problem, get a faster automatic. Maybe a newer accord, maybe a bimmer, a acura tsx?
The difference in a new auto really wouldn't make sense. The extra gas cost really isn't that much more, versus a whole new auto which would be my second car. I'm probably going to sell my RSX and Civic some day, and look into a new automatic TL.