Using 2010 21" iMac as second display
Yes, since OS X 10.9 Mavericks there is a function called Target Display Mode or short TDM. In short: this is a manner to use your iMac as screen for another Mac, not for PC's. You can connect your mac by miniDisplayport or Thunderbolt cables.

Unfortunately you need an 27-inch iMac Late 2009 or Mid 2010 to use mini display port, or an iMac-21 inch mid 2011 or iMac 27-inch Mid 2011 to use Thunderbolt. 

All the requirements can be found here: http://support.apple.com/kb/HT3924

You surely can use a Dell screen, if you've got a minidisplayport or thunderbolt port, then you can buy a minidisplayport to HDMI/VGA/DVI... Best way is to check out which connections you've got on your iMac and then search for an transformer. (You just can use an minidisplay cable in an thunderbolt port). A workaround could be an USB display adapter, but those don't have the best preformance...

Good luck!