Solo para empezar, vete mirando si tu fuente tiene un conector de 6 pines para alimentar la gráfica... porque si no, ni siquiera podrás enchufarla.
Las fuentes "genericas"... suelen ser bastante petardas, y la 4850 pide una potencia apreciable y estable en un rail para ella solita
Corto y pego de la pagina oficial de ATI los requerimientos:
ATI Radeon™ HD 4850 System Requirements
* PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
* 450 Watt or greater power supply
with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
* Certified power supplies are recommended. Refer to
http://ati.amd.com/certifiedPSU for a list of Certified products
* Minimum 1GB of system memory
* Installation software requires CD-ROM drive
* DVD playback requires DVD drive
* Blu-ray™ playback requires Blu-ray drive
* For a complete ATI CrossFireX™ system, a second ATI Radeon™ HD 4850 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per graphics card (included) are required
A partir de ahi... tu mismo