I'll try to clarify on that.
1. INFR2 is never coming. The Inferno team scrapped the original mod, and are working on a new version, INF SCP. It has the same basic storyline, with cooler graphics, better missions (we hope, and not that the originals were bad) and generally updated stuff.
2. The order in which you should play the mods is really INFA (or INFA Standalone, when it's released), INFA2, INFSCP, INFSCP2, etc.
3. You can play the original INFR1 with a retail FS2 install, but it is suggested that you get the SCP patch, as this fixes some bugs, and it's the SCP, so it automatically makes it better. Not to confuse you, you need the FreeSpace Open .EXEs to use the SCP patch.
4. The general consensus is, yes. Most people are being able to play FS2O on Vista 32 and 64. Post your specs though, so that we can see if you're worthy. (although if you ave 4/8 GB of RAM, I doubt that the rest of your PC lags behind much
5. What he said.
Also, here is clarification on what Inferno is:
INFR1: Inferno Release 1. The original Inferno, released in 2003.
INFR2: The sequel to R1, canceled in favour of INF SCP.
INF SCP: Same basic storyline as INFR1, but uses the improved SCP engine, as well as having many graphical enhancements. Unreleased.
INFA: Inferno Alliance Storyline detailing what happens in Sol after the Lucifer is destroyed. New ships, and pretty graphics.
INFASA :Inferno Alliance Standalone, does not require any FreeSpace 2 files to run, includes INFA with improved graphics and missions, as well as extended storyline.
INFA2: The sequel to INFA, will run on the INFASA engine.
Sol: A History: Mod by Blaise Russel, one of the finest FREDders alive. Same basic premise as INFA, but from a different person. Many people have trouble getting it to work.
Steadfast/Dreamcatcher: I mention these cause you may hear tell of them in the forum, they are both mods based on INFR1, side stories.
Solar Wars: A buggy piece of ****, not even worthy of the title Alpha Release.
Order in which they should be played: INFA/INFASA, INFA2, INFSCP/INFR1, INFSCP2