As j_garcia implies, the particular speakers will make a difference in what is required, as some speakers are more efficient than others. Also, the acoustic characteristics of the room are relevant, as some absorb more sound than others, and the placement of the speakers is relevant, as well as the listening position. It is also important to know what kind of music one listens to (as some is more dynamic than others), and how loud one wants it to be.
If we consider a typical example of a speaker of, say, 90 dB @ 1 watt @ 1 meter efficiency, this means that if you feed it 1 watt and sit at one meter, you will hear sound at 90 dB, which is loud. However, you are likely to sit further back, and so it may be more like 85 dB at your listening position (the exact level, of course, will depend on how far you are from the speaker, as well as the acoustic properties of the room). To add 10 dB, you need 10 times the power, so at that listening position, 10 watts will give you 95 dB. If you don't have serious peaks, and if you don't want to go deaf, really 10 watts is likely to be enough. 100 watts would give you 105 dB, and 1,000 watts would give you 115 dB (assuming, of course, that the speakers are capable of this and will not be destroyed with such power). (If you double the power output, you add 3 dB to the output. Notice that all of this means that a slight difference in output will make very little difference in dB output, so that a slight difference in power is likely to be unnoticeable.)
As a general rule of thumb, I like to have about 50 watts per channel for a stereo, though, as stated above, that may be more than is needed. Of course, if one has inefficient speakers, or one listens at literally deafening levels, 50 watts may not be enough.
It is also worth noting that there is no disadvantage in having more power output capability than one needs (except, of course, to one's budget and the likely use of additional power for a higher electric bill).
