For starters, it's not "L Ray", but rather iRay (it's been around since 2003)  There are hardware specs at the Nvidia site as well as other info

It is not necessary to have a fancy graphics card, it can use CPU only rendering.  If you have a high end graphics cards you will get slightly better performance.

I have a GTX 750, but don't use it for rendering acceleration because most scenes that I do are too large to load into card memory.

You need to have 4.8 installed, it comes with iRay gratis.  You can still use 3Delight if you like because iRay doesn't support certain shader networks.

I haven't looked at any tutorials for DS, because it doesn't operate substantially different in DS that it does with any other program.
There are differences in implementaion of MDL, but that only matters if you are going to write your own shaders

Just to be correct, it is Iray. LOL. (Sorry I couldn't resist.)  The biggest differences between 3Delight and Iray, in DS, is lighting. Since it is unbiased physically based, light it like you would light something in the real world, and set your tonemapping like you would a real world camera.