Mealz - i guess it depends on how you look at money. I would ask myself this question -
How big is $300 to me?
It always seems to me with homerecording....that most of the stuff I want ends up somewhere AROUND this range...so...200-500 range..really.
The things you would want to look at upgrading - soundcard as mentioned, mic pres as mentioned...getting more mics..etc.
So - is it really worth it? Like you said, you are 15 - do you have a lot of serious projects going. IMHO - you are better off not getting too gear hungry at this point...becuase you probably can't afford it. Spend your money wisely. (and cash in on those birthday and christmas presents).
The NT1 is a fine mic...and there is plenty you can do with what you have. ESP in terms of piano..guitar..vocals. Read some info on what mics to use for which occasion...there is a good thread in this Mic forum that harvey gave some great info in. it's called "How does polar pattern effect which mic to use" - or something.
I say all that to say that knowledge..not gear is really going to be the difference in a good recording and a crappy one. Say you've got three years to really hone your skills and learn how to be resourceful with the gear you have - and then when you turn 18..you can probably afford to start buying some nicer stuff...but you will really appreciate it..and know how to use it. ya dig?
Anyways...gear you might want to look at..or ask for as presents:
Soundcards: Midiman Audiophile 24/96, Mia, Delta44 or Delta66...
Preamps:
Midiman Audiobuddy...the Studio Projects tube thing.
Mics: Check out the cheap Behringer ECM8000 Omnidirectionals. there is a lot of info about those on this board as well. For 70 dollars you get a pair of full capable omnis that would probably be a great learning tool for you. And would probably do great things with your piano (also get you into "matched" pair stereo miking)
Other: Maybe get a cheap keyboard with a MIDI interface, so you could start learning MIDI programming/gigasampler...stuff like that. If you are a keyboard player i bet you could do a lot with a little cheapo.
Monitor Speakers: This can get expensive fast (500-1,000/pair) range...but, in time you will find them essential. I'm sure your yamahas will be great for you for a while. heck, if you can get good mixes out of them maybe forever.
Pick your software and learn to be fast with it - hang out on these boards and learn all that you can. later
-Wes