Yep- I think the 5570 is probably pretty close to the most powerful low profile card you can get. Its great on power draw, and is reasonably powerful, though you'll definitely need to lower details, and probably resolution to play well on it.
my pc came with Nvidia driver, and do I have to delete them? or it shouldn't a problem even if I install new ATI driver?
You go into device manager and remove your device (GPU) this will delete the drivers. Then you shut down and install your new card. On boot up you will be running on Windows original VGA driver but that is the time to install the driver for the new card.
Definitely remove the nvidia drivers, and also- I can't remember the last time I even put a driver disk into my PC- its all downloading the latest version from the manufacturers website. Catalyst 10.3 is your friend:-) http://game.amd.com/us-en/drivers_catalyst.aspx
HDMI is a different port from DP. Displayport is a less common port right now, and is hard to find on displays, so HDMI is probably better for you right now, though Displayport does have some advantages that hopefully will get more useful in the future.
guys, I have one more question, I really appreciate you guys' help.
my network adapter is nvidia nforce10/100/1000 mbps ethernet. if I remove nvidia driver from display adapters, then there won't be any change to network one right? because they are from the same brand I was just worrying about it. sorry im nub.
yeah- just make sure you remove only the Nvidia display adapter drivers, and not the ethernet drivers. You can even specify that when running Driversweeper- to leave ethernet drivers and remove the display adapter drivers.