Satellite P300 run perfectly with Win7. I have installed it last year on brother’s P300. It runs really good and stabile. Performance is also very good.
In my opinion you should not even think about WXP. Install Win7.
is it possible to upgrade directly WITHOUT making a CLEAN install
Generally speaking it is possible but I prefer always clean installation so I strongly recommend you to make the same.
If you need help and some assistance with Win7 installation (right install order) let us know.