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.
The extra 6 pin 12v connector adds a better ground and source for the 12v draw of the card, but the connection is the same 12v from the PSU as before. I would try it and see if the system ran stable especially under load. A single 4870/4890 will draw more power than the 4850, but not so much more. Here's a comparison of pwer usage 4850-4870. 4890 not released when this test was made.