I prefer to scan everything as 16 bit files, it gives me the most amount information for color, contrast, saturation, etc. adjustments. Then convert to 8bit.
I'd save the file out as a 16bit file, close, then open in CS5.1 Camera Raw and use the noise reduction feature. Another option would be to use NIK Define2.0 or another 3rd party noise reduction software.
Noise Ninja, Topaz DeNoise, are some off the top of my head. Most have free trials.
I'll ask the obvious: Have you tried the Filter - Noise - Reduce Noise feature that Photoshop provides? It can produce quite good results.
Also, opening your scanned TIFF through Camera Raw avails you of its luminance and color noise reduction facilities, which are also quite good.
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