Ok, after owning my 240 for over a month now, I've done some thinking...and I've decided that putting vast amounts of money into my car will not be worth it, because I figure I'm going to be getting a new car in 3 years or so anyways. However, I'm going to be replacing a lot of stuff soon, like the spark plugs, filter, and muffler(the stock one is starting to rust out).

So I want to replace them with more quality parts.
I'm planning to get ngk platinum or iridium spark plugs and new wires and put in a K&N air filter. Considering modifying the airbox/resonator.

Should I get a cone filter instead of a regular one that fits in the stock airbox?

When my stock muffler finally dies, I'm hoping to get a better muffler (or new exhaust system). What would be a good muffler to upgrade to?
I prefer dual tips and one thats not very loud. Just a little louder than stock (a deep, low hum, not a roar)but not ones that draw too much attention. I've heard that magnaflow makes semi-quiet exhausts and boosts performance slightly.
Any cat-back exhaust systems like that?

Basically, I'm looking for quality parts at decent prices with marginal performance boost.