Since I am in the United States, I am not too familiar with the European money system, but by using a convertor, I see that the amount is equivalent to about 50$ if you are using the United States money system, so I am sorry to say, but 50$ just wont get you much.
How about upping that amount to 100?
If you CAN go up to 100, I would shoot for the Geforce3 Ti, or if you cant find that, the Geforce MX 440 AGP 2x/4x or AGP 8x.
If you can pull a miracle and get up to 150, you can get the Geforce FX 5200 ULTRA by ordering it online.
Just look around, search NewEgg, BestBuy, etc. and hopefully you can find something suitable.
Good luck with the upgrades!