I agree with those seat suggestions. The Radian will arguably last the longest both RF (40 or 45 lbs, 40?? inches), FF (65 lbs, 18" top harness slots) and in temrs of expiry (8 years), but is also the most expensive. The Scenera is a great budget seat but very basic, no EPS foam etc and only has a short harness (14.5" top slots) and 40 lb limit for FF. It is 35lbs RF and has a decent height shell, and has a 6 year expiry. The Avenue is similar to the Scenera but with EPS foam and more padding. The CA is 40lbs/36in(?) RF and 50lbs/18"THS for FF, and 6 years expiry?? (or is it 8 like the 3in1s?? anyone?). MOst kids outgrow seats by height before weight so it would be very similar to the Radian. But I think the Radian is narrower as well.
All these seats should fit fine RF in your Civic unless you are tall, especially if you install a bit more upright. I have had the Radian RF in my '07, I think I tried both in center and driver's side, and I fit comfortably to drive - but I'm 5'2".