Even if the company with which you are interviewing has a casual business dress code, do not assume that this means you are welcome to dress in this manner.

A gentleman is expected to dress as follows:

Suit: A two piece dark grey or dark blue business suit will be ideal while a white long sleeved dress shirt is recommended.
Shirt: Men should choose light colored dress shirts.
Tie: Plain colored dark blue or dark red 100% pure silk tie is good.
Tie Accessories: A classic silver tie bar help you keep your tie in place so you don’t appear nervous.
Shoes: Black or brown well polished shoes is the best choice.
Socks: Wear dark socks (black will be ideal)
Belt: It is recommended that your belt should match the color of your shoes

A more comprehensive material is attached.

U re right. When i went fr an interview at one of the new generation banks. We were all waiting to be called, suddenly, the H R manager came out & picked few applicants that were on suit & said openly that others were not serious. I was lucky, i was well suited.

Of course, it's best to be best prepared.