To answer your question, the PSU cannot be too strong, because the motherboard draws power as it needs it, rather than taking the full potential of the PSU constantly, so that it cannot receive more power than necessary
The PSU having alot of power does not damage the system,the system draws the power it needs only.You should always have a 10-20% more power than the system actually need to perserve the PSU.
You should look at the quality and Amps and Watts,in that order.
A good quality 500w PSU is better than a 750w PSU.
Having more Watts doesnt necessarily mean more power,the power comes from the effectiveness of the PSU.For example,a good high quality PSU can reach or near its max W outtage whereas a bad PSU with claim having XXXw while it can only deliver much less.That of course mean that the +3v,+5v and +12v rails have less available power that what the manufacture states.
The relationship between W and Amp is a very close one has in W turn into amps to power the PSU cords that power your GPU,HDD,DVD-ROM,etc.
To sum it all up,look for a good quality built PSU,these deliver good Amperage at lower W and their components and safety features can keep your system safe,always buy a PSU with a bit more power than you need so that your PSU with be running at i.e:70% at max load intead of almost 100%,the more loaded the PSU is the shorter its life-span is.