To my knowledge there isn't any sort of internal clipboard in AHK. And using commands such as ControlGetText are going to be buggy if you have anything other than text, or if you want to preserve formatting.
This is the way that I've usually accomplished this sort of task.
#+c::
oCB := ClipboardAll ; save clipboard contents
Send, ^c
ClipWait,1
SplashTextOn, 160, 20, Clipboard, New Clipboard Copied
Sleep 2000
SplashTextOff
Return
;Now, pressing Control-V pastes the new text, and can be used as often as desired
#v:: ;And pressing Windows-V copies the old back to the clipboard
ClipBoard := oCB ; return original Clipboard contents
ToolTip, Old Clipboard `nRestored
Sleep 1000
ToolTip
oCB := ;Clears variable
Return
This is bascially what lintalist was referring too. But by using the variable ClipboardAll you should be able to save anything to the clipboard, including formatting, images, etc.
Check out http://www.autohotkey.com/docs/misc/Clipboard.htm for more info on this command.
Also, shout out to pajenn on the AutoHotKey forums for the seed of this code. http://www.autohotkey.com/board/topic/39280-get-selected-text/