I agree it should work in your system, but it may not be an ideal match with existing components.
What resolution is your monitor? Getting the full potential out of a 1GB card such as that 4850 would require 1680x1050 or better resolution. Lower resolutions tend to only need 512MB cards. Also, at lower resolutions, the CPU is often the limiting factor in overall performance, but that limitation becomes more and more dependent on the GPU as resolution increases.
Also, does your power supply have sufficient 12V Amperage to power this card? Can you post it's specs or make/model?
Edit: LOL @ typing a comment the same time as everyone else...