So, basically i have a dell optiplex gx620 (small form factor) and it has an 80 gb harddrive in it. I want to upgrade to a 1tb harddrive but i don't know if my PSU can support it. If not, would a 500 gb harddrive be good?
The storage space of an HDD has very little relevance to how much power it consumes. It's possible that a newer, larger HDD may actually use less power than your older 80G. If you want to take a conservative approach try the WD Caviar Green series of HDDs which are specifically designed to use less power.