Since Litecoins seem to be immune from ASICs...is it worth building a GPU rig for mining?
Seems like the difficulty keeps increasing every other day.
Not necessarily immune. But the building requirements are quite different. Basically, what is more of a "pure" computational algorhithm with BTC is a RAM intensive Algorhithm with LTC.
So, you could definitely build it. The question is at the moment: Would it be profitable? Would people pay so much money for it, that it makes sense?
And at the moment, I doubt that.