Your current video card Nvidia GeForce 9400 GT is ranked in the chart at number #108 and rather outdated while supporting DirectX 10.
Your mainboard is from 2009. (Would have made a good combo with the 4800 Series from Radeon.) I doubt you can play FH2 on highest settings. The current system requires about 350W estimated.
The Radeon HD 6670 is some low-cost videoard pushed on the market in April 2011, it is ranked at #44, slightly below the Radeon 4800 Series, which have been introduced in 2008 already. It is supporting DirectX 11. The advantage on this new variant is less power consumption compared with the 4800 Series at slightly less performance. Estimated required wattage for this system is about 350W. Your video performance would increase significantly, without leaving much reserves to play future games above low graphics level. You could play FH2 on high settings, with few restrictions such as low AA and AF.
The Radeon HD 6870 is some rather new videocard released at the end of 2010, ranked at #12 in the charts. It is the high-end (luxury) version of the 6800 Series. The Radeon HD 6800 Series and the latest Radeon HD 6900 Series are based on the same chip, making it a proven and reliable chip. Good chances for excellent driver support in the future. It is supporting DirectX 11. Estimated required wattage for this system is about 450W.
On long terms the Radeon HD 6870 will do excellent, while the more cost efficiency Radeon HD 6850 is probably sold out already. If you can get your hands on a Radeon HD 6850(ranked#14), you could expect a reduction to 139€ while the system would only consume about 400W. The Radeon HD 6670 on the other hand is almost of the same low cost type, as your current videocard was when you got it. The lifetime will be limited, while the graphics of future games will be average.
Limitations:
The general problem here is, that you got Windows XP which will use only 4 GB memory. 4GB will be considered outdated for games soon. Your current Windows XP supports only DirectX 9 and exludes you from DirectX 10 and DirectX 11 effects on all these videocards anyway. Those DirectX 10 and DirectX 11 updates are rather little, but usage requires often high performance videocards to be used in notable proportions.
Considering that your mainboard was rather cheap, while being combined with an AMD CPU, your looking for cost efficiency and a good long term investment.
Conclusion:
If you consider to upgrade the RAM to 8GB and the OS to Win 7 soon, while having at least a 450W power supply, the Radeon HD 6870 would however suit well to the Athlon X4 CPU. They'd probably get outdated at the same time. Though I can't really predict the future, it looks like a fine combo.
If your current power supply is only about 350W, it might be cheaper to buy the Radeon HD 6670, to be replaced rather quickly by another low cost card. That would get you around a power supply upgrade.
The Radeon HD 5700 Series mentioned above is overpriced at about 128€, considering its high power consumption (400W system total) at rather poor performance.
Disclaimer:
Please don't complain about wall of text, as the information given was a bit limited. Don't make me accountable for possible mistakes on my part. Always consider the possibilty of fatal errors. You might be forced to deal BIOS and software upgrades when combining older mainboards with new videocards.
Edit:// corrected price for 5700 Series.