I'm not certain that spending potentially a few hundred bucks for the sole purpose of shaving some wattage off your system makes great financial sense. Unless your system runs under load pretty much constantly, it would take quite awhile just to break even from whatever power savings you might see.
That being said, here are some thoughts:
In spite of its age, the Phenom II series still competes well with some of AMD's newer chips. In fact, the more I look at the benchmarks, the more I'm sad that I got rid of my old 955-based rig. It runs hot, but for single-threaded applications, it performs well. Now for something like Handbrake or Photoshop on the other hand, you'd likely see a nice little boost with something like an FX-6350 or 4350.
If you really want to get serious about power consumption and efficiency, then what you want is an Intel chip. Consider the i3-4130 with its 54 W TDP, compared to the 125 W TDP of the AMD chips. You'd likely sacrifice some general productivity performance with the i3 over the FX models, but then the i3 might be a better gamer.
Last thing I'll say about CPU's: Although AMD is posting some impressive general productivity numbers with their FX chips these days, I still like Intel for their (generally speaking) vastly superior single-threaded performance. If I were going to buy a new CPU tomorrow, I'd get an i5-3570k.
Thankfully, the situation is a little more black-and-white when it comes to your video card. The 4890 was a great board back in the day. I ran a single 4870 (and then later dual 4870's) for quite some time with good results. But that was then. If you're doing less gaming nowadays and want something that's energy efficient, I'd get a Radeon 7770. If you're concerned that it might not provide quite enough graphics horesepower, then a 7850 would be a good compromise.
To summarize: if you're just really unhappy with the performance of your Phenom and are willing to spend the cash to upgrade your platform, cool. I think the 6350 especially would serve you well. Otherwise, stick with your Phenom just a little longer and wait for Steamroller. In the meanwhile, double your RAM to 8 gigs (it's so cheap these days) and grab one of those video cards I mentioned.
Just my .02 - and I hope that helps.
Gigabyte Z68X-UD3H | Core i5-2500K @ 4.6 w/Corsair H60 | 8 GB G.Skill DDR3-1600 | VisionTek Radeon 6950 2GB | WD VelociRaptor 74GB + WD Black 1TB | ASUS Xonar DX | Bose Companion 3 series | ASUS DVD-RW + Lite-On BD-ROM | Corsair Professional Gold AX850 | Antec Three Hundred | Windows 7 Home Premium x64 | 52" Samsung LCD HDTV @ 1920x1080...FTWI need more cowbell.