when i am normally forging i wear just my sunglasses ... as i know that these have good UV protection.. (prescription)
when it comes to the protection factor i was told that you have to wear something to cut out the IR that comes from the forge.. anything that is between youe naked eye and the forge will help with that ..
(though i still havent found anything around here that claims to cut any other amount of IR radiation)
and then you need to protect against the UV radiation .. which most good quality glasses will do.. though, as Reg said, some glasses are just tinted glass and will provide diddley for protection.
i think that if you are going into large amounts of welding .. or needing to glare into the forge for large amounts of time .. then the torch glasses are probably the way to go.
but again .. this is just what i have found out from talking to people here...
D.