how playable is a PC game in win2ds?

It depends on how good your computer is, as well as the connections between your ds and the computer. I've never tried to actually play a game with Win2DS because the response was too slow. However, if you've got a really nice pc with a good router, fast connections, and your close to your router, it could probably work out.

if you wanna play maplestory doesnt work

what about GTA: SA?

What does 'playable' mean?

a decent frame rate... like not to much frame skipping