I want to upgrade my dell vostro 270s desktop. can any expert answer my following questions..!
currently I'm having 500GB hard drive installed.
If you're willing to sacrifice the optical drive, you can replace it with a 3.5" hard drive in a suitable bracket -- other than that, the only way to add a drive is going to be externally by USB.
You'll be limited to low-profile and low power video cards -- there's no room inside for a full-size card, nor will the power supply support anything resembling a "latest" card.