Motherboard is the same as in my Medion - PCI-E graphics slot.
The PSU may lack a PCI-E power feed, or lack the npower and connectors to use a molex to PCI-E feed - in which case, a meatier PSU would be needed for any card dependent on a power cable - I think X1600 models may not need them, but that would not be a huge upgrade, though significantly up on an X600.
If you want real power, I think you are going to have to go to the X1900 series and a new PSU.
The X850XT is also wotrth a look, a pretty potent member of the "pre-X1k" series, but that means it's PS2.0 and not PS3.0, and most of the driver development is now on the "X1k" series.
http://www.ebuyer.com/UK/product/111193/rb/22860263458
This MIGHT be a drop-in replacement, can't tell if it has a PCI-E power feed though, but pretty sure an X1600 can work without. This is 512MB, MIDRANGE, X1k series, if you want more, then it's an X1900/1950 model and almost certainly a new PSU.