1. I assume I can install the bitcoin gold wallet on the same PC as my Electrum wallet, correct?
Yes. You should be ok.
2. At bitcoingold.org there is a section saying "Check your address balance before block 491407". Where do I find this address?
That's your Bitcoin address that may contain BTG. You can check to be sure there is something available to "redeem".
3. If I sweep my BTC from Electrum does the BTC remain there and only the BTC Gold is swept into the gold wallet?
I'm trying to understand what wallet are you talking about. Electrum is only for Bitcoin. If you spend your coins with Electrum, you will only spend Bitcoins. If you spend your BTG with a Bitcoin Gold wallet, only your BTG will be spent.