lizardguts :
Whats the advantage of the x series psu over the 620w psu, is efficiency that important? (Other than the obvious full modular and slight gain in watts) Also if I wanted to go for a nicer one would it be better to get http://www.newegg.com/Product/Product.aspx?Item=N82E16817139010? (Due to the extra 100 watts just in case I wanted to SLI in the future)
That link doesn't work for me.
A few advantages of the X series over the one you posted would be the Fully Modular interface along with the 80+ GOLD power efficiency rating.
Having a fully modular interface allows for better cable management and easier cleaning of your PSU fan. You could just unplug all of your PSU cables, unscrew your PSU from your chassis, then procede to dust the fan out.
Efficiency will have an impact on your energy bill, however, this may or may not be of relevance for you. The 80+ BRONZE certification on the PSU you listed is already good, so 80+ GOLD may not be worth the price if energy bill isn't a big concern
The fully modular interface, however, is the selling point of the X-Series.