HD 4850 installation problem

I had an ATI HD3200 (integrated) but now i have an XFX ATI HD 4850. With both video cards i have the same problem, i can`t install them right. I tried diferent ways as told in wiki.cchtml.com but i have the same problem every time. After installing and rebooting the monitor turns black and says "Out of range". Maybe i'm missing something, i don't know. I'll thank every help i can get. This is driving me crazy!! ](*,)

Ah, i'm using Kubuntu 9.04

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My hard: CPU: AMD Phenom X4 9550; mother: ASUS M3A78-EM; video card: XFX ATI Radeon HD 4850 512Mb GDDR3; Hard disk: Seagate 320GB 7200RPM; monitor: ViewSonic 19'' VA902b