editing pages problem

[zap]

I have a problem with editing pages. They are defaultly located in data/ folder under 755 privileges. But I can edit them only if i set privileges to 777 which seems to me a bit much. WHat should I do?

When I am in debugging mode with privileges 755, php throws an error:
Warning: SimpleXMLElement::asXML(../www_root/data/pages/mapa-stranky.xml) [function.SimpleXMLElement-asXML]: failed to open stream: Permission denied in /home/www/sastyle.sk/www_root/admin/changedata.php on line 171

Warning: Cannot modify header information - headers already sent by (output started at ../www_root/admin/changedata.php:171) in /home/www/sastyle.sk/www_root/admin/changedata.php on line 174

data fles privileges are set to 644

thank you all for your help