No more than $ 1.50 over spot for GENERIC silver (rounds,bars). Things like American Silver Eagles (and other government issued coins) carry a larger premium, especially now that the US mint has suspended production. If you choose to buy 90% junk silver, you should be able to get it at spot or even slightly below. (About 13x face value right now).
As for the gold coins, if I were buying I would aim for the government issued coins (American Gold Eagle, Canadian Maple Leaf, South African Krugerrand, Euro Philharmonic) and would try to find them for about $ 50 over spot if I could, maybe $ 70 or so over spot if I had to. Keep it mind that if you buy fractional gold, you will pay a bigger premium.
Also keep in mind, that premiums do change because the physical metals market doesn't always stay in sync with the paper COMEX game. So you have to survey the market when you want to buy so you know what to expect AT THAT TIME.
I would try the forums over at
http://www.kitco.com/ then click the Gold Forum button at the top,
or go to
http://goldismoney.info/forums/
Sometimes people who go to dealers or coin shows on the weekends will post threads letting you know how big premiums are at the time they go. Just make sure to read the date on the thread to make sure it is current information. If you can't find the info you want, just ask them on the forum.