Have to spend 15€ on Ebay, help needed!

For some reason I got a 15€ PayPal coupon on Ebay out of nowhere. That's nice, but I don't really know what to spend it on and it will become invalid in a few days. There are three games that look interesting to me, but I'm not sure about any of them:

1. Rust. Looks like a nice game, but it seems to be mixture of Garry's Mod and DayZ. Messing around with silly stuff and spawn killing other players for equipment. I tried Garry's Mod and didn't like it, it was simply too pointless for me. Is there any actual gameplay worth mentioning in Rust (other than messing around and laughing about silly looking naked people)?

2. Borderlands 2 GOTY. I played Borderlands 1 and it was somewhat ok, but in the end it was just Diablo converted to a 1st person shooter. Not bad, but not great either. Is Borderlands 2 much better than 1? Or is it exactly the same thing just with a few new missions and maps and updated graphics?

3. Far Cry 3. I would buy this immediately, but my PC is not that great. I only have a Core2Duo e8400, 4 gb ram and an ATI 4890. I don't mind reducing a few settings (guess I'd have to do that in the two other games as well, especially in Rust), but if I have to play on lowest settings possible with 20 fps then it's not much fun. Would it run ok with medium settings and medium resolution or can I completely forget about it?

Any help would be appreciated.

Oh, and of course I have an awesome game to give away.