Don't sink any money in cables. Period. At 15 and with a limited budget, there are better uses for your money. Cables aren't one of them.
I've read the threads you've recenly posted and think you would be better off acquiring source material (LPs and CDs) than hardware. You've got a great system. I'd hate to see you cash out on hardware only to listen to the same source material over and over again. Have you decided on a turntable yet? That's a better use of $500 than cables.
Take your girlfriend out to see a band. Buy a guitar. Get out of the house for a while. Trust me, I remember what it's like to be 15 and obsessed with music. I still am (19 years later), although a wife, 2 kids, a job and an MBA program kinda get in the way.