Assuming that your projector has one Component and one HDMI connector, you need to switch only among PS3 and HTPC HDMI signals/cables. You connect Wii to Component and you can use passive switch for HDMI.
If your projector has only one HDMI or only one Component connector but not both, then you need to switch 3 signals and convert (either Component to HDMI or HDMI to Component). If you need to convert, then you cannot use passive switch and it will cost you more money – you then need not only a switch but also a converter. A switching and converting AV-receiver may be the most (cost) efficient and smooth solution. If you already have a good solution for sound and you obviously don’t need up/down scaling (as you indicated in the other thread here) then you only need to use the switch and convert functions in the new receiver which means cheaper receiver – you then don’t care much for its sound amplifier part. Count your video inputs and outputs and needed conversions and buy the cheapest AV-receiver from one of known vendors that gives you all what is needed.
Returning to general switch discussion, there are mechanical, electromechanical and electronic switches. The first 2 are passive and the last could be either passive or active.
A mechanical switch works as your light switch in the wall does. It doesn’t switch signals, it switches the leads. Therefore it can be used for any signals – you only need the appropriate number of switched leads in the switch and right connector types. If you are a DIY-type, it is quite simple to build one yourself using off-the-shelf components. The cons: no remote control, not possible to convert or scale signal. The pros: cheap, simple, switches any signal, reliable, DIY, doesn’t affect the signal in any way, bidirectional (important for HDMI and to some extent for VGA) and for all practical purposes it is electrically invisible.
An electromechanical switch works similar to a mechanical switch, but the switching is done with relays instead of with a manual rotary knob. This means that remote control is possible, but it will be more expensive than the mechanical equivalent. Good AV-receivers contain a lot of such relays – you can actually hear the clicking when you change inputs/outputs.
A passive electronic switch uses active electronic components (op-amps etc) to switch signals. When appropriate components are used (fast and broadband enough) then it can switch any signal too. However, video, both analog and digital, require a lot of bandwidth and very fast electronics (better resolution – more bandwidth and faster electronics) so such switches are usually rated only to a certain resolution. Such switches may be quite cheap if produced in sufficient quantities. Remote control is easy to implement but the switch does affect the signal. It also requires an external power supply. Most often these switches are unidirectional – no good for HDMI, VGA requires special treatment (strapping and software control). Bidirectional are more expensive.
Active electronic switches are actually more of a processor or controller than a simple switch. They “understand” inner workings of a signal and often reproduce that signal on the output. Therefore they are made for a specified (specific) signal or signal types. Remote control is possible and often done as well as automatic input sensing and switching. These switches can be built to convert and scale the signal. They can be quite expensive, especially in the pro market.
Currently I’m using Joytech AV-switch. It has 4 input groups and 1 output group and it is operated by IR remote (or by buttons on the switch). Each input group has RGB (full scart), S-video, Composite video, Stereo-RCA and Toslink (audio) connectors. The switch connects one input group to the output group without any conversions. I haven't tested it, but I suppose it would be possible to use scart to switch Component and/or VGA signals. The switch is unidirectional, passive, and electronic. I used this switch to switch both RGB and audio. I just got a new AV-receiver, so I’m using it to switch audio while RGB is still going through Joytech – the receiver cannot handle RGB, only Component, HDMI, Composite and S-video. The rest of the world doesn't get it that we in Europe use RGB not Component. Cheaper and better!