There is no upgrade for you for 150 Euro that would be worth it. A 200 euro card would only increase the performance of games by a few percents... Crossfire is not an option either, as your second PCIe slot is x4 (instead of x16). Save more money, or wait a year or 2 to get a good boost for 150 euro.
its over budget but I would at least get a gtx 460 1 gig / hd 6850 if you are upgrading from a 4850. There are cheaper options but you will not see a big increase from a 4850 which is still a decent lower/middle end gpu by todays standards imo. I just got a 6870 and am thrilled with its performance/price.