so if i want to get more hz (let's say the default is 1200, i want 1300-1350) all i have to do is to buy a stronger PSU?
the current one is 850w gold.
I would measure what the PSU draws right now. Overclock one blade, measure again and calculate if your PSU can handle it. PSU's are most effective when used intensively (but not over their limit of course). If it is not enough than you can buy another one. Or use an extra smaller one for one or two blades.