What it comes down to, is the PC you have isn't very good.
AMD FX-4100 3.6Ghz - Barely passable for gaming. Will do very badly in CPU bound titles like BF, GTA IV, and others. You're essentially limited to 40ish FPS at max settings even if you had an infinitely powerful GPU. FX simply isn't good for gaming.
4GB DDR3 RAM - Passable, though 8GB is standard these days. Newer games are more RAM hungry, and you do see small performance increases going to 6 or 8 GB.
ATi Radeon HD 4830 512MB - Slightly below average for its time, its outright aged now. This is probably the biggest bottleneck in the system, and combined with a weak CPU, is significantly holding back performance.
Really, to play at max settings, you basically need to tear the system apart. A GPU upgrade will allow medium/high, but that CPU will probably limit you to that going forward.