420W PC&C PSU + Radeon 4870/4890?

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.

My PSU - 420W PC & C 80-Plus Certified:

My System:
AMD Phenom II X4 940 125W cpu - 1.35 volts core @ 3.3GHZ
4GB (2x2GB) Corsair DDR2 800 RAM - 1.9 volts
Biostar 790GX mATX board
Asus DVD/CD writer
1TB Hitachi 7200 HDD
1 USB mouse
1 Keyboard through native purple port
1GB Radeon 4850
3 120mm fans (2 through the board)
1 140mm fan

Sorry for being a total n00b in regards to PSUs. I have never had a PSU fail & I don't want to start now.

Any problems, concerns, or thoughts?
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More about 420w radeon 4870 4890
  1. 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.
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