What would be a decent & cheap upgrade for me?

It depends on what you are going to be doing with this computer.

What is the maximum you want to spend? Can't really give an answer until we know what the budget is.

under a 100$ for the mobo, and perhaps 100-150$ on cpu?

Well, gaming whenever I can.. Like I said, my card gets bottlenecked now  

If the main purpose is gaming I would go with Intel. You are going to need a new motherboard, new CPU and new RAM. Are you keeping anything from the old computer or are you trashing the whole thing and starting over. If all you are getting is a new motherboard, RAM and CPU I would go with an Ivy Bridges I5 (220 dollars), Z77 motherboard (120 dollars) and 8 Gigs of RAM (35-40 dollars.) That will come to about 380 dolllars.

AMD Phenom II X4 965 Black Edition Deneb / Gigabyte GA-970A-UD3 AM3+: $198

The main thing is: Getting rid of the bottleneck on the 660   I assume that's the cheapest i5?
Quite an expensive set

Looks pretty much a bottleneck-eliminating upgrade! But I do not think newegg ships to holland?  

No it's the most expensive I5 because it has the unlocked multiplier. There are cheaper I5's but they can't really be overclocked. I would not go with Phenom II. While it's still capable of playing games it is starting to show it's age. You are going from a old processor to a slightly less old processor.

EDIT: Also if the I5 is still to expensive you can go with the I3 which will be pretty close to the price of the Phenom II and at stock clocks the I3 will be faster.