The psu wattage you need is primarily gated by the graphics card/s you will use.
Here is a nice chart which includes typical other components:
http://www.realhardtechx.com/index_archivos/Page362.htm
Keep your psu.
If it needs to work overly hard, you will hear the cooling fan spin up.
When it comes time to buy a new psu, buy only quality.
There are good and bad units from most makers.
Safest, I think is to buy Seasonic.
The new focus line as a very good one.
I have no problem overprovisioning a PSU a bit. Say 20%.
It will allow for a stronger future graphics card upgrade.
It will run cooler, quieter, and more efficiently in the middle third of it's range.
A PSU will only use the wattage demanded of it, regardless of it's max capability.
The main reason for a high efficiency psu is to reduce cooling fan noise; efficiency has little bearing on quality.
Modular is nice, but you are going to use most of the leads in a well sized psu anyway.