if there is any city in Poland that where artists (painters, writers, sculptors, etc) gravitate more to.
Kraków, Warsaw, Wrocław, Trójmiasto, Poznan and Katowice, approximaely in that order.
Krakow also isn't that cheap but I guess it also depends on the area of the city.
In Kraków, Nowa Huta and Podgórze, both very interesting neighborhoods, are the places favored by young artists now. Kazimierz has become rather trendy, and artists are moving on from there. In Wrocław, Śródmieście is the place for young artists. In Trójmiasto, Orłowo is the place for serious artists. Not cheap, but beautiful. In Katowice, old, "scenic" neighborhoods like Nikiszowiec are popular among artists.
I know moving to a village would be cheaper but you'll be a bit isolated and away from any arts community.
Au contraire. Thriving artist communities in the smaller cities, especially in Kazimierz Dolny and £owicz. Other good places are Cieszyn and Jelenia Góra. A lot of artists seek out quiet villages, especially in Bieszczady, Suwałszczyzna, Kaszuby and Ziemia Kłodzka. I have an artist friend who lives in Wisła, which on the map, and in real life, looks pretty isolated (but gorgeous), but has excellent transport connections, including a direct express train to Katowice and Warsaw.
Or is it more hip to move to a place like Berlin or Paris?
Lots of better places than that for younger artists on a budget. Dresden Neustadt, for example, has a fantastic art scene. The festival there in June is a must-do.