& what about amps yar a psu with 30 amps should be bought for 5770 or a psu with 18 amps?
18 amps could do it for your PC , I'm just not into low amperage, low quality power supplies, I like to leave more room for upgrades.
The current office system builds I provide consist of
No videocard , not needed for these applications
Antec EA380 / Corsair 400 , depending on which has the better deal at the time ( either of these would work for you as well )
Prime 95 causes a draw from the wall of ~82 watts ( AC ), that's roughly a 65 watt ( DC ) load, if you have a psu that can maintain 80% efficiency at such low loads.
The 5770 has been shown to draw ~106 watts ( DC ) when stressed with Furmark.http://ht4u.net/reviews/2009/amd_ati_radeon_hd_5700/index9.php
Add 'em up, 171 watt load on the psu, just a bit under 18 amps when stressed with the power viruses.
You really don't want to run a psu close to it's max all the time ( obviously wouldn't unless you were running just the PV's, but gaming can bring you close )