Help... I have GAS

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Peace be on to you dudes,Thanks all for the advice and help in the past. I have  a millermatic 200, and  I recently found a good deal on 3 tanks which i've pruchased 2 are 100% argon and 1 nitrogen.  Since i'm new to welding, i'm not sure what the nitrogen is used for, or if i need it at all.  I know a lot of people are saying you need argon sheilding gas 75/25co2 (right now i'm using flux core wire) but would love to switch to sheilding gas (too much hassle with flux core)  Would 100% argon work better on mild steel? or would i be able to exchange the tank for a mixed tank (75/25) or would 90/10 work better ?  I'm using this welder just around the house, and currently making a gate out of steel tubing, so i'm trying with your help to make a good choise on what gas i need for the application i'm using and probably will be in the future.
Reply:Pure argon is most commonly used for tig welding. 75/25 (argon/CO2) is a basic gas mix for short-circuit mig welding of mild steel. Once you decide to mig weld with spray transfer, you must adjust the percentages a bit, less CO2 is needed, perhaps 15% or less. Sometimes a bit of oxygen is added when mig welding stainless steel. http://www.lincolnelectric.com/knowl.../migweldss.aspIf you use a spool gun to mig weld aluminum, the argon would be correct.  http://www.millerwelds.com/education...rticles10.htmlAs far as nitrogen goes. Try google...http://www.google.com/search?hl=en&q...=Google+SearchCity of L.A. Structural; Manual & Semi-Automatic;"Surely there is a mine for silver, and a place where gold is refined. Iron is taken from the earth, and copper is smelted from ore."Job 28:1,2Lincoln, Miller, Victor & ISV BibleDanny