As far as a power supply, the games played to an extent have an affect in power usage, but it's more the hardware the games are played on. The SIMs are pretty light compared to FPS's with the latest and greatest graphics. Games like Crysis 3, Titanfall, Far Cry 3, Metro: Last Light would all push the hardware much further than the SIMs would. Your computer as it is presently should handle 450W, however nothing is stopping you from going higher. Of course going higher is more expensive, but then if you end up building a new system later that's more powerful, then you might not need to buy a new supply. Just be sure to go with something with a good name. Corsair, Seasonic, and some Antec, are good supplies. To calculate power requirements, I use an online calculator. It has drop downs where you tell it your system configuration and it spits out a number based on the published power requirements of each component. The number it gives is a worst case scenario, so for GPU and CPU, they are loaded to 90% in the calculation.
Noise is subjective, some people are more sensitive to it. Some people game with headphones and don't care how loud their systems are. Some people sleep in the room their computers are in and want them as quiet as possible. In your case any new fans are going to be better than just having the one you have. I would get the quiet ones, but it's really up to you. As for location, you will have the one at the back (from your second picture) and one on the side panel. Looking at the picture of the side, it looks as if the holes drilled in the plexi double as ventilation and mount holes. I couldn't see holes dedicated for screwing the fan to the side, so if that's the case, you'll be able to tune the position of the fan to exactly where you want it. I would place it directly above the graphics card. That way cool air will be pushed down to the graphics card. As I mentioned before the one in the back should be an exhaust fan and the one on the side should be an intake fan.
As for the vacuum, it will get most of the dust. However in the most crucial areas such as the CPU heatsink and GPU heatsink, it will leave dust down inside the fins. With the CPU, you may be able to remove the fan from the heatsink and get the bristles of the brush down in between the fins and get most of the dust. The GPU is going to be more difficult.
As for the APU vs CPU question. AMD's APU's are probably the slowest processors for the desktop environment. They are not as fast as either the Athlon FX series from AMD, or pretty much anything Intel has. Unfortunately the APU platform has a unique socket, so you can just pick up a FX and install it, that would require changing the motherboard as well. Now that doesn't mean they aren't good, they're just not as good. I don't know exactly which APU you have, so I can't give you specific numbers on performance. However the SIM's isn't the most demanding game out there on the CPU. What matters the most (to some extent) is the GPU. I had a look at the Wiki for the SIMs 3 and it's expansions and your present card doesn't appear to work for many of the expansions:
Wiki SIMS 3 Requirements
If you were to consider upgrading the graphics card, I would suggest the 650Ti or the newer 750Ti. My daughter has the 650 and it's a great little card. She plays Team Fortress 2 on it and it keeps up quite nicely.
As for the heatsink, you have a stock heatsink. They are engineered to cool "sufficiently" under most "normal" ambient temperatures. However if your apartment gets quite hot, then the APU is going to stay warmer as well. Any heatsink that supports sockets AM2 / AM2+ / AM3 / AM3+ will work on your motherboard. If this is something you are considering you can't beat the Hyper 212 EVO for price / performance ratio. It'll be leaps and bounds better than what you have. I've checked your case dimensions and it's not too tall for your case either. However this may require you to remove your motherboard to install because it has a backplate. I couldn't find the manufacturers website for your case (only retailers) so I can't tell if there is a cutout on the motherboard tray so that you can access the back of your motherboard without removing the motherboard. Some cases come with a cutout, some don't.
I hope that helps. If you have any question about a particular part, post the links in this thread and I'll give you my opinion on it.