What is a good amount of money to bring to enjoy a fun evening here with friends? =) Cash?

Depends on what you plan to do. Buying games? Bring a couple hundred. Playing arcade games? $10 is fine.

At least 20, 40 if your going to get some beers

I think $10 lasted my friend and I about 1-1.5 hours. Depending on how many friends you have $20 should keep you fairly occupied for a while.

There's an ATM that will pay out in 5s and most games are cash! $20 should pay for a group of 4 people over an hour