Seems as if the Speedlite 420ex can't keep up with the speed of the 50d. Many missed flashes.
Any Speedlite!
yes
Yes most defenitley.
Yes, but with some limitations. The Canon 480EG was designed as a TTL flash (through the lens metering) but also has an External Auto metering Mode. It can be used with any camera that has a PC sync connection. Don't use the "Shoe Cord E" that comes with the flash unless you are using a Film Camera with TTL flash metering. It can be used with Digital cameras provided you use a PC sync cord to connect the flash to the camera. Use the camera in manual mode and match the settings that you want to use with the flash.
It does not work with my xsi - the picture is over exposed
Yes, It will.
A recommended flash for the Canon T3i is the Canon 430EX II. This flash is about $270. However, honestly, any Canon flash will work well with the camera.
Any standard hot-shoe flash should fit the XSi, though check the flash sync voltage to be on the safe side. Using a Canon EX-series Speedlite will give you a variety of additional features; other flashes won't be able to synchronise at shutter speeds faster than 1/200th sec, and won't work in live view mode, either, for example.
Actually no they won't. I don't konw alot about canon cameras. but one thing I do know is that it prabably won't work. Just get a Canon flash or a Sigma flash.
Absolutely. The 430EX was designed to be a cheaper speedlight option for Canon SLR users. It was especially designed to work with the 5D which doesn't have a built in flash. If you buy a 430EX it'll interact with your 30D, 350D or similar EOS SLR to help with white balance and focusing as well as giving you more professional looking flash photos. The flashgun will read the zoom length of your lens, how far it's been zoomed and other info from the camera and automatically adjust it's power and throw to provide the right flash angle and strength for each photo.
I don't know how long ago you asked this question, but the 540EZ is VERY old. It's pretty much obsolete now. Go for the 430EX or the 580EX. They're a decent price, and work very well. If you want more power out of it, the 580EX Mk II is your best bet. ;)
Any remote flash can be triggered with a long cord in the camera body sync socket or hot shoe mounted sync cord (which satisfies the criteria in your question). To be truly wireless, I can think of two options. 1) A small, hot shoe mounted slave flash triggers the remote flash when a sensor attached to the remote sees the slave go off. Downside: Anyone else's nearby flash will likely trigger your remote, distance is limited, slave may not work in high ambient light. 2) A camera mounted radio transponder signals a receiver connected to the remote. Good range, ambient light not a factor. Downside: will cost about $400.