It might work, can't be 100% sure with OEM-motherboards. My girlfriend's 2007 Packard Bell PC (different motherboard then yours, but still similar) won't take HD 5xxx cards, it does however work with HD 4xxx cards and I even got a Nvidia GT 440 to work. Since your motherboard at least still has support there might be a BIOS update for it that makes it work with newer graphics cards, so if the HD 5670 doesn't work at first, try the latest BIOS update, if that doesn't work just get a GT 440.
You're right but although those two were sold at the same pretty low price (44,00€ = 38.85£ = 62.92$) on a Finnish ebay-like site the HD5670 was sold without install CD and manual so I chose the other one. Besides, after GeForce 6150SE it probably feels like being in heaven anyway. Thanks for information and those charts, though!