The 620 is the newer iteration of the card. the 520 is "last years card" So to speak. You will get better performance with the 620.
Thanks! Going with the ECS GT620C-1GR3-QFT GeForce GT 620 1GB
I'm planning on buying this card but couldn't find too many reviews for it. Can you please let me know if the fan is loud on this card? Also, how is it since you've had it? Were there any unforeseen issues with this card while you've been using it? I'm planning on using this card in my htpc build to play 1080p media connected to a receiver so I am also wondering if there are any handshaking issues using hdmi. Thanks