I wouldn't consider the 5450 even for a computer this old, simply because it's too weak to play any games. If you really want a cheap upgrade you should go for something like a hd 6670 or maybe if your budget allows it a 750ti. The 750ti will most likely be bottlenecked by the rest of the system, on the other hand the 6670 should be fine and should let you play those games you mentioned at low to maybe medium settings (medium would be possible if your monitor is low resolution which it might be considering the age of your pc). You might want to grab a psu with that video card too, something like a corsair cx430 should be fine, maybe even a decent 350w antec/seasonic/corsair psu. Now, it would be nice to know the exact specs of your system so we can tell for sure if the upgrade even makes any sense or not. If it runs a core 2 duo and 2+ gigs of ram it should be fine. Hek, I ran just cause 2 on a pentium 4 3.4ghz with a 8800gt at 1280x800 on medium settings settings with no problems and the framerates were decent.
EDIT: I youtubed some pentium gameplays of the games you want to play and it looks like arma 3 and dayz might be okay but BF4 is definitely not going to play well even with a good gpu