Okay, so for the price range you're looking at and the current place in the AMD/Intel competition, You may want to look at getting an Intel motherboard, with an ATi graphics card.
I suggest grabbing yourself a socket LGA 1155 motherboard with some form of Sandy Bridge, Core i7 chip. We're already spending about 400 euros at this stage, but you want to dish out that bit of extra cash for the motherboard, so you can upgrade without choking up too much in say, 2 years time.
Once you've picked yourself up a processor, we'll need to focus on the memory, generally you should be fine letting out 80 euros for some nice 2x2GB sticks, or a 4GB (maybe even 2x4GB). As long as you've got 4GB, you're fine, you can upgrade another time if needs be.
So were at nearly 500 euros now. You have two options really, option A would be to grab yourself an SSD (about 64GB) and fairly low spec terrabyte drive for dumping your data and older games on. This would cost you a fair bit, say around about 120 euros. So it might be wiser just to get a fast high spec 1TB drive (7200rpm) for around 80 euros. This would leave the rest of the money for the graphics card.
With about 150 euros to spare you have to make a decision. Based on which motherboard you've chosen, if you've gone for the convenience of the competition, you'll probably be best off getting yourself a 6850, or a 6870 for the bit of extra cash. I read a comment earlier suggesting that some games might need more memory. I shouldn't imagine any games in the near future will require anything more than a gigabyte. Also remember that big numbers of memory does not mean fast performance. A 512MB 4850, could hammer down a 1GB 8500GT any day, the memory isn't a huge factor.
I'm no tech guru, and I'm talking only from experience of having built a few nice gaming rigs in my time, but these are the steps I'd go through. Make what you want of it.
