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:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

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 mobo)
1 140mm fan

Any problems, concerns, or thoughts?