A few years ago there was a craze of adding a 'PSone screen' to PC's. They took in RGB information, with sync, which could be taken from the VGA port of a two-output gfx card. As the screen was much lower resolution many used a small app for graphics cards that allowed them to change the settings in the cards bios. Wasn't actually that difficult, but the quality of 'console screens' is pretty crap - at least compared to todays high contrast, high resolution TFT's.
The DS has a lovely screen, as does the PSP. Alas both have no controller, and take 24-bit colour information, as well as horizontal and vertial syncs, and bit clocks. To use either or those screens with your PC (for a smaller auxiliary display) you would need either VGA to digital converter, plus your own display controller (read: very difficult/complicated), or a USB microcontorller, with an FPGA for the LCD controller, as well as software for the PC to determine what the LCD displays (also difficult/complicated).
However, both of those are the true DIY appoaches, which although VERY cool, isn't always practical. There are many high quality TFT displays which already have built in circuits designed to accept composite video (yellow RCA connector) which can be taken from s-video, or VGA I believe with a relatively simple circuit. Think, in-car DVD players with mounted displays. They contain the LCD controller, as well as any video decoders needed.
Beyond your abilities? I never like to say
I tend to choose projects with misplaced ambition, get frustrated, but stick with it and they can take literally years to complete when worked on now and then - thats time consuming and expensive and its the price I pay for being delusional. But a great compromise would be some kind of TFT 'module'. They have sizes/resolutions similar to the DS display. but much moree easily interfaced to either video equipment, or a PC's gfx card (or a second smaller, cheaper, gfx card that works on PCI).
If you want a touch screen, I believe, unlike modern ipads/ipods, the DS uses a resistive touch screen overlay. - You can buy those seperately and add them to any display you wish providing its the right size. A USB controller for this touch screen panel can also be brought relatively cheaply, plugged into the PC, and you get to write your own software
Thas how I would do it anyway. Still some work, and perhaps slightly more expensive that just buy a DS screen and FPGA, but a lot less effort, with much nicer 'hardware hacking'.