Is there any difference adjusting in game settings from low to ultra? Does ultra or high make things more realistic like real life like?
Yes there is a difference, exactly what the settings do differ from game to game as the low/med/high/ultra settings are normally abstractions of more complex options to make it easier for people to use. The basic principle is that quality and fidelity of the 3D world increases with higher settings.
Some games aim to be more life like but not all do, sometimes effects are used purely for aesthetic appeal, not everything that looks pretty is realistic. When it comes to life-like games you need to understand that 3D graphics are always just approximations of real life phenomena, developers might implement several different ways of approximating these kinds of phenomena and some look better than others but typically need better hardware to run.
A real world example is say shadows, there's lots of ways of approximating shadows in a game, some are basic but run on old/slow hardware, so these are used for low settings, and others are much more accurate and realistic and are used for Ultra settings. This range of settings exist to allow people with older hardware to play new games by turning down the settings. As you turn up these settings they start to lower your frame rate, so it's a trade off and you typically need to tweak your settings to find a good balance between quality of the graphics and smoothness of the frame rate.
The hd 5450 is quite an old video card now so you'll be stuck using low settings for modern games, in fact some modern games may not even run on that card at all. Upgrading would allow you to play BFBC2 in high settings and everything will look better and be more crisp and clear.