My wife likes arcade tye games (those that take up the whole Windows screen. They are even flakier under Parallels than they are in Windows - one has the cursor offset from where she sees it. One asks for a video card compatible with Direct X 8.0 or higher. One directed her to Indeo for a codec - which costs $15 without being able to try it first. I ran that game on my computer and it didn't ask for anything. (although my buggy Windows machine aborted when I closed the game - device driver error). What are the video limits of my wife's 24" iMac?
Currently parallels does not support 3D or DirectX. At the moment it enables you to run windows with 32mb of video ram so that you can have a nice screen resolution. It is rumoured to be available soon in the next big release but until then its no games for you To see what graphics card your wifes iMac has installed click on the apple logo in the top left corner, then click "About This Mac" now click "More Info" and finally select the option from the left that reads "Graphics/Displays" this will provide you with the information you require.