I know these types of questions are posted on here all the time, but every system is a little different ... blah blah blah. I'm upgrading from a 1GB Radeon 4850 to the 1GB 4870 or 4890. I think that I should have enough power because the 420w currently runs the 4850 without a problem. However, the 4850 runs on 1 6-pin connector & the 4870 and 4890 runs on 2. I used a PSU calculator and it says my system only needs 340 watts. However, I want to make sure that my 420W PC & C psu should be sufficient to power my system.
It should work but its cutting a bit close. Get a 4pin molax to 6pin pcie which should come with the card, but do check. Your pc will peak out at probably 350w max overclocking cpu and gpu, but dont go mad with the clocks cuase your pushing the psu pretty card.