Thanks guys for your comments. Images really look much better with flats.
For those who still don't know how to take them I would recommend to read
the easiest explanation on how to do it I've ever seen. It's written by Paul(1ponders) and this is the third message from the top in there.
Also there is one more image I took yesterday. I guess it is my best shoot of Orion nebula. Stacked with flats too :-)
[9x180sec light frames at ISO800 + 2 dark + 2 flat]. Equipment used: Celestron C8 with 6.3 focal reducer, Canon 300D modded with Edmund glass. Stacked in DeepSkyStacker and post processed in Photoshop CS2.
Autoguiding: Atik 1C, ED80, GuideDog.