...with a 19" resolution, a 5750 is really the best you can get for your proposed 120€. Games like Mafia II will run just fine and Call of duty too.
even If you had a 22" with 1680x1050 res, it would still run fine but with a few less intensif feature like AA etc...
You should also consider older cards such as 4850 (like others kindly suggested) BUT only if its a real bargain AND remember that the 4800 series are hot and consume a lot more then the 5000 series. This also reflects on noise and heat in your pc case.
For your PSU, you don't need a beast, but don't go for a noname!, corsair makes proper psu's for relatively cheap prices and in your situation, a VX450/VX550 are a must if you want something good for a few years to come.
Finally don't under estimate the potential of unlocking your fourth core!
I own a 720BE myself and a similar gigabyte board, unlocking the fourth core gave me a 15-25% boost in many games, GTAIV, AVP, and many others, games get more and more multithreaded nowadays , so the more cores, the better
So, a good psu, a 5750 or alike and unlocking your fourth core, should give you a nice boost and good for a few years to come i guess.
As for LFS, well, a 5750 could even run it with 3x19" with decent settings , if your passion goes that far in the futur, never know, lol.
Hope this helped you out a bit.