dell inspiron 580 slim PSU upgrade from 300w

Hello, 

Thank you for taking the time to read; 

I am upgrading my i3-550 to an i5-760 CPU thus making me have to change my heat sink. I already have an upgraded GPU so I will need to get a PSU with more max power 400w ect. I ran CPU-z for some information but cant find if i need an ATX or TFX PSU. Also a lot of Power supply's im looking at doesn't have the fan near the power cord, rather (on top or bottom) will this affect the cooling of the system internally?