Building gaming rig, PSU advice?

I'll be building a rig using an i7 3.5 @ 4.5 w/8GB RAM, not sure about the GFX card yet, whatever I can get for 180 euro. I have a 620w PSU from my last build, which I would have thought was enough, but when I look at similar systems and the components they use - they often have 750w PSUs. Do I really need a 750+w PSU, or will I chance the 620w?