With CoolEdit 2000, there's a Noise Reduction function that comew with it. Alas, with Sound Forge, this is not an includeed faeture so you need a DirectX plug-in to do this. (At least, this is how it was up through version 5.0; I have not upgraded to 6.0 yetand so I'm not aware if this was added as a part of the standard package). You might also be able to do it in some manual way using EQ; maybe a Sound Forge expert out there can help.
Basically the process is to "teach" it the noise's signature by isolating a small bit of tape hiss when nothing else is going on (that is, what would have been dead silent passages if there were no hiss). Then it processes the entire audio file by essentially subtracting this signature from the whole file.
Depending on how "pure" (that is, constant, not changing much over time) the noise content is, this method can work wonders, or you can end up with a flat, muffled sounding result.