Looking to upgrade. PSU Good Enough?

My current system is as follows:

Case: Antec 300 with 5x green LED fans
Mobo: Asus M4A78 Plus
RAM: 6GB PNY DDR2 800 pulled out of an old prebuilt.
GPU: Sapphire HD4870
PSU: Corsair 550W
CPU: Phenom x4 9850 BE with ZALMAN cooler
HDD: a single 640GB 7200 RPM drive
Soundcard: Soublaster X-Fi Xtreme

This fall, I am looking to possible upgrade my CPU and video card. I would like to upgrade my CPU to a Phenom II 945 and I'm sure I could power that just fine with my current setup. However, concerning a graphics card upgrade, I am not sure what to pick.

I was thinking of getting a Sapphire HD 6850 but am unsure if I could power it. Preferrably though, I would like a 6870 but again, I don't know how much stress this will cause my PSU.

If neither of these cards is acceptable by my PSU, are there any GPU upgrades you guys could recommend that I could safely run?