All I wanna know is what I can make from say a decent GPU card (300 Mh/s)... 
Almost nothing.
Difficulty is currently projected to increase at approximately 25% every 2 weeks. At current difficulty, your rig will earn you 20 mXBT per day. If you leave it running forever, it will earn you at most 2 BTC, probably less than 1 BTC.
forever, meaning ∞?
After 2033 all the bitcoins will be discovered, (= 21 million BTC).
You will not be able to discover new ones.