I recently purchased the SBLive Platinum 5.1, so I can attempt to answer your questions.
The outputs on the card itself are front speakers, rear speakers, and digital/analog out. The digital out can send an AC-3(Dolby Digital) signal directly to a speaker system that can decode it (such as Creative's DTT3500). Also, it can be used as the center/subwoofer output. I have mine hooked up to my stereo with 5.1 inputs using the front, rear, and digital/anolog outputs.
Yes, it has 6 speaker outputs.
It has a built in decoder, so if your DVD software can output the 5.1 signal, it can decode it and route it to the correct speakers. I do not have a DVD drive, so this information is directly from the manual.
It doesn't do any encoding (that I know of), but if your DVD software can output the raw 5.1 signal, the SBLive can route the raw signal out the digital output to allow the external receiver to do the decoding.
The LiveDrive IR has MIDI in, MIDI out, SPDIF in, SPDIF out, Optical in, Optical out, Headphone jack with volume control, 2nd microphone jack with volume control, and the IR receiver. The remote can control all of the Creative software that I have tried to use it with. I have not gotten it to work with winamp yet, be I have it configured to where I can start winamp using the remote. It can control audio and DVD's, as long as the software you are using is designed to use the remote.
5.1 does not make a difference when playing games unless the game includes 5.1 encoded sound. i.e. - Quake sounds the same as it did on my Aureal SQ2500.
Haven't noticed any performance gains/losses.
If you are talking Hifi equipment, couldn't tell you what speakers are best. Creative's DeskTop Theater speakers are the only ones designed specifically to interface with the Soundblaster Live 5.1 (according to them).
Hope that answered a few of your questions.
If you need website reviews:
http://www.gamecenter.com/Hardware/Audio/Sblp51/?st.gc.fd.hw.i
http://www.tbreak.com/reviews/hardware/others/sblive51/sblive51de_1.asp
Jeremiah