I used the Diamond GC 500 and the Elgato Game Capture HD for my let's plays.
https://www.youtube.com/user/PowerPlaid/videosAll my videos uploaded used the Diamond GC, It's ok but does not support the resolutions of N64, Genesis, PS1, and a few others. Not a bad lower end card if you get it cheep enough.
The Elgato has a great end result but is a bit finicky, It handles files poorly, saving it in 2 spots on your PC and sucks up a lot of space, and it occupationally screws up, I lost a recording because it did not save it for whatever reason and it flips out a bit if your PC runs out of space. But it is kinda worth it, it is very easy to set up, plug it in, set the input, and record and the footage looks beautiful, and worked with every system I tossed at it, Genesis, N64, PS1, PS3, Wii U, Wii, NES, ect.
I'm happy with it, I can post some screenshots of footage if you like.