Unwired goes into a lot of detail (fluff text) on how all the matrix stuff works, including SIM systems.
As I understand it it's like this:
SIM data (Sensory info, including touch, heat, smell, sight, sound, etc) is the key to the Shadowrun VR (and thus Riggin) experience. Getting that kind of info into a person's head is what you use Trodes or a Direct Neural Interface (DNI, like a Data Jack or Implanted Commlink) for. (Note that a datajack is a better link, but there is no game penalty for using even the NanoTrode Paste... which is said to be a crappy link.) That link also provides some basic input and output. Still, if you want to use VR of any kind you still need a SIM Module. A SIM module is kind of like a modern graphics card. It handles Simulated Sensory Info, such as a SIM recording, or the output of a Rigger adapted vehicle and then pumps that through trodes or a datajack, into your brain. That means a rigger that has jumped into a car feels the car as if it was his own body.
A Control Rig has nothing to do with any of that. A control rig is an extra implanted OUTPUT from your brain back to the car. Normaly when you use VR or even just a DataJack to type up a text message, you are thinking at your computer. A Rigger jumped into a Car feels like he is the car and he mentaly directs it to drive. The Control Rig is described as tapping into the
middle brain which controls Motor Control. The idea being that riggers are actually controlling a physical thing instead of negotiating a virutal world of metaphores with a persona that may not even have limbs. A rigger with a Control Rig doesn't think at the car, he just moves, making it a much better form of control. Thus, a control rig doesn't make you feel more like the car (because it has nothing to do with sensory info) it lets you control the car more intutitively.
In general all the VR related stuff work in one of a handful of ways:
Trodes, Datajacks, etc all provide a connection between various parts of the brain and a computer. While it is said that nanotrode paste is a poor connection and a datajack or implanted computer and sim setup is the best, the game does not note any benefits or penalties to using any option here. All these options seem to focus on connections to sensory input and output of the brain. Of course in order for the computer to do much more then provide limited input/output, the computer will need to be able to processes digital siganls into sensory input. For that you need a SIM Module. SIM Modules only convert digital signals into sensory. If you want to conver outputed sensory data into digital signals you need a SIM Rig. SIM modules and SIM Rigs are attached to a computer, and don't have to be implanted. Unwired has a lot of details on SIM encoding meathods and codecs... if you're interested.
To enhance a VR experience (and gain a bigger dice pool) you can approch things from a few ways:
Make the SIM sensory input stronger: Hot SIM Modules and
SIM Accelerators(Unwired) do this by having their safety limits removed and by speeding up processing of the digital into sensory info respectively. They work in the Commlink and thus don't have to be implanted. Note that a SIM Accelerator (Unwired) is a great add-on for both hackers and riggers.
Make a Stronger Connection to the Brain: A
SIMsense Booster (Augmentation) does this. It adds extra direct plugs into the various parts of the brain in order to get signals into the brain more efficiently, and thus boosting their strength. It must be implanted. It works great for both Hackers and Riggers, and it stacks with a SIM Accelerator to give you 5 initiative passes!
Provide better output from the brain/body back to the computer: A
Control Rig does this by providing motor control info from the brain to the computer, allowing for a more seamless control of rigged vehciles. It's of no use to a Hacker because the VR world of the matrix isn't exactly real... unless your in a UV Node.
Enhance the Brain to be faster/smarter which has the side effect of making the person a better hacker:
Encephalons (Augmentation) &
Math SPUs (Augmentation) both do this and provide benefits for techicanl actions both in and out of the matrix. They don't do much for Riggers, but they can be a great benefit for hackers.
Enhance the source of the input (IE: more sensory data from the vehicle you are rigged into): Suprisingly there isn't too much about this in the vehicle rules from Arsenal. Mind you, sensors in general are kind of a weak spot in the 4ed rules. Generaly any sensors you put on a vehicle are senses you get when you rig in, but there is one vehicle/drone mod that takes this a bit farther:
Touch Sensors (Arsenal) provides sensory output from the vehicle for the sensation of touch by placing small sensors everywhere. You don't feel like your rollerskating down the road, you feel like you're running barefoot down it. You feel gunshots, or a hand on the door, etc. This extra input provides a +1 bonus to handling and some other checks, but also makes you feel a lot more pain when the vehicle/drone takes damage. You might also consider taking
Improved Sensors (Arsenal) and then filling the larger amount of sensor slots with even more sensors, but that won't help you get any dice pool boosts.
For a near perfect rigger setup you need:
A good commlink with a Hot Sim Module & Simsense accelerator.
A Simsense Booster (implanted).
A Control Rig (implanted)
A Drone or Rigger Adapted Vehicle with Touch Sensors.
That's 5 initative passes with a +5 bonus on most jumped in rigger actions. You feel like your drone, move it the same way you move your body, and act at the speed of thought. It's the nearest thing to being the drone itself.