If you have €100 per month then you have more then enough for a used ATi card with Dx11 and 1GB. The cheapest HD5450 (1GB) i found on ebay cost $60. I'm very sure you can find it even cheaper somewhere else.
I strongly believe you can own both a good 1GB card + a new power supply and still have 20-30 euro left, out of a €100 ;)
The HD5450 won't be able to play TDU2 even on lowest settings, it isn't much better than your original 8400GS. And even HD5670 isn't the best choice, I suggest getting HD5750/HD5770, or maybe HD4770 if you still can find it. The 4xxx series doesn't have DX11, but it really doesn't matter with TDU2.
So my suggestion for lamborg is: Get HD5750 and a new power supply, then you should run TDU2 quite well. :) It isn't really wise to buy any lower than 4770/5750, because the price difference is quite little and you get much more for your money.
http://www.overclockersclub.com/reviews/sapphire_radeon_hd5450_512mb/10.htm
http://www.tomshardware.com/reviews/radeon-hd-5670,2533-7.html