"The Kelvin (K) temperature is an approximation of the color of the light (the higher the bluer, the lower the redder). It is supposedly the color of light that a black body (a piece of metal) will radiate if heated to that temperature, but it's not very precise when applied to lamps other than incandescent. It does not say anything about the spectrum of the light.
So, the difference between 10,000K and 3,000K degrees shows up most as the color of your tank. You can't say if a bulb is good for plants from just the color temperature. Daylight is about 5500K color temperature."
Quoted from http://www.geocities.com/Heartland/Hills/2637/faq.html#Kelvin
So, take into account the above, the 'K' rating is not very accurate for fluorescents. Using the daylight value of Kelvin, 5500K, I would think the fluorescents around this 'k' would be enough to grow plants. A more important consideration is not the 'k' value but the output or intensity in lumens/lux.
Therefore, a 'K' rating of 6500k would be great for growing plants especially if you can get 4 watts/gallon. Im not sure if I would go below 5500K though. I believe that most cheap cool white tubes found are hardware stores are 6500K (so who says expensive 'plant-gro' tubes are better?).