The Boxster has to be the best 3rd car around. My Brother has 2 sedans and a Boxster and this is his claim. It depends on what you're looking for. A 5 year old high end sports car is a very different purchase than a recent year 4-seater under factory warranty. You will get some bills on the Porsche but they average out over time. The car won't break down, but when you do regular stuff like brakes and service intervals the immediate price will be a hit. I'd be surprised if a RX-8 is going to beat a Boxster on the track. You could put more power into it with some mods, but a 50/50 distribution, mid-engine and Porsche suspension is tough to beat if you have equal drivers in the cars. If you're a modder and you enjoy adding HP or tourque to a car then the Boxster is hard to do that with. On the other hand, if you're well-maintained older car ever does go the RX-8 can take over duties.
You can do better than that deal you saw on the Boxster so I'd research that a little before making your decision. If you're patient you will come across lower mileage cars in the 2000-2003 MY range before long. My 2000 base has 50k miles on it and is in excellent condition but I'd be surprised to get $18k if I sold it. My friend's similar 2000 S with ~30k miles just sold for $20k. That was a "friend" price but probably would have been at most $21k on the market. Take a look at these two links below. I bought mine from these guys years ago and they are great to work with. I've been to their place in Houston and it's the real deal. They'll sell Boxsters for about $500-$1000 below their listed price. Not saying you need to buy from them (though my next car will likely come from here) but they are a good gauge for what a nice example should sell for if you can find it locally.
http://www.expomc.com/invsys/details2.php?id=6935
http://www.expomc.com/invsys/details2.php?id=7127