If it definatly does have RCA inputs, the sound quality will definatly improve. Using RCA inputs you have a physical connection for your audio, where as using a tape adaptor the audio is converted from electrical, to magnetic(?) then back to electrical, each time, loosing quality.
Of course it is going to sound like a cassette when you are using a cassette adaptor, because that is what it is, your simulating a cassette in the player.
As for investing more money, unless you need a special adaptor to utilize your RCA inputs the only things u should need are a decent set of RCA cables.
My plans and progress on my install http://www.mp3car.com/usersites/hornet