What to upgrade in a PC?

Hi,

sofar my PC specs are the following:
AMX Phenom II x955 (4x3.2Ghz)
16GB Ram
Windows 7
onboard Soundcard
AKG K272 HD Headphones
Johannus Opus 25 Console

Now i'm thinking about upgrading my PC and would like some advice, what would be most worth it/give the biggest improvement? In quite some articles I read, that a good audio-interface makes a big difference?
And if so, which one could you recommend, keeping in mind, that I plan to stick with Stereo-output as im using Headphones only.

A better Soundcard will yield better sound. A better processor (and thus new motherboard and possibly RAM) from Intel will give higher polyphony. If the latter is not an issue, get a better Soundcard (be it internal or external).

What best suits your needs also depends on budget.

We are "wintering" (I say this tongue in cheek because what we are escaping is 70 degree days and 50 degree nights) in Puerto vallarta for three months and I brought along a casio previa 88 key piano to use with my laptop. I had to leave my dongle with my HW setup as students will use it for practice while I'm gone, so I'm using my laptop with the free edition and earphones. (At some point i need to buy a second dongle so I can stop carrying it back forth between office and living room) . The point of this being, that I wasn't happy with the sound so I just ordered (using the Mexican branch of amazon) a Focusrite Scarlett Solo to use and just got it setup yesterday. I'm very pleased with the improvement it made in sound. It is a minimal unit but will feed earphones and one stereo pair of speakers. You are only paying for one unused mic input and is USB powerd.
John