If you're not wanting to do full-screen or 3D gaming, then VNC or RDP or screencasting will probably fit the bill. It will work best where there is very little happening on screen - you'll probably get up to 1/2 - 4 FPS depending on WiFi and how much is being updated on the screen.
VNC and RDP will give you a crisp, pixel-for-pixel picture that is pretty close to real-time, and are generally pretty light on resources. There are a multitude of clients for iOS devices; windows comes with an RDP server (look up how to enable Remote Desktop Connection), and I believe TightVNC and UltraVNC are two free VNC servers which are pretty popular.
Screencasting (such as with Join.Me or Livestream) actually generate streaming video - you can get much higher FPS with this method, but it is extremely processor intensive (you're encoding video in real-time) and there is also a second or more of delay. Any fine text will probably not be readable because the output resolution will be fairly low.
3Any solution will probably have 1/2 - 4 FPS so this will depend very much on the game you wish to play. 3D/FPS games will likely not play well. – Darth Android – 2012-10-02T20:02:05.790
And source on that? – jcora – 2012-10-02T20:03:25.223
58+ years using Remote Desktop and VNC and screencasting utilities, any of which are going to be your options here. The iPad does not have a video in connector, which means you can't just stream raw video from your laptop's monitor out. RDP/VNC can get feel much faster, but that's because they only update small sections of the screen. Full-screen updates take a while. 20fps at 1080p is ~120Mbps of bitrate. Wifi can't handle that in real-world scenarios. Movies use compression to make that work (~10-20Mbps), but that is not realtime. – Darth Android – 2012-10-02T20:59:20.497
1The particular source for the FPS reference in my first comment was my attempt to play World of Warcraft over VNC using a gigabit wired connection, with WoW running on my gaming desktop and playback happening on my laptop. – Darth Android – 2012-10-02T21:04:46.063