Well, the only experience I've had with this kind of stuff is downloading ROM's for a virtual gameboy and some MP3's off of Limewire. And I haven't done either in years.
The GB ROM's actually got me to buy several games. The music MP3 generally didn't help much as I only downloaded music that I just couldn't find to buy anywhere, older music usually.
For games, yeah I much prefer demos. DA2's demo saved me $50. Without demos it's non-professional reviews and Let's Play vids on YouTube. If you can't tell if you like a game from that then you just aren't trying hard enough.
Now as far as knowing whether or not your PC can play the game ... well that is different. There are some programs you can run (some free) that will basically 'test' your gaming PC and let you know what are you lacking. I had just upgraded mine a year ago so it would play Skyrim fully maxed out with HD textures, so I wasn't really worried about this game. My brother however has an old PC that's been having problems running WOW and other older games, but he doesn't have much problem with this game. So overall this game is not very demanding, plus if you get the game setting set right it makes things even smoother.
But, as a final note, I have to say that you really shouldn't acquire anything you know is being done illegally, whether you know you'll get away with it or not.
