Windows 7 RC will be avilable for download from MS to everyone from the 5th May, along with legal keys that will work for a year (I beleive).
The following is from an email sent to all those that downloaded the windows 7 public beta
On Tuesday, May 5 (PST), the RC will be available to everyone via our Customer Preview Program. As with the Beta, the Windows 7 RC Customer Preview Program is a broad public program that offers the RC free to anyone who wants to download it. It will be available at least through June 30, 2009, with no limits on the number of downloads or product keys available.
As to why bother with vista, for me the factor that was decisive in my own personal move from XP to vista was the need for a (fully released) 64bit OS, with comprehensive driver support. Whilst there is a 64bit version of XP it does not have anywhere near the driver support that vista 64bit has.
As to why I needed a 64bit OS, it was quite simply when I moved to an intel i7 platform, with tri channel memory support. It made sense at this point to go for 3*2gb ram chips giving a total of 6gb ram rather than 3*1gb for a total of 3GB ram. To get the full benefit of 6GB ram required a 64bit OS. In my personal view 64bit Vista is a better option than 64bit XP.
I have been using vista now (64bit home premium SP1) for around 4 months and have no real issues with it compared with XP. It does offer imo some benefits over XP although it is slightly but not massively more resource hungry from my experience to date.
Windows 7 is looking very good, from what I have seen from the public beta and I will be trying out the public RC of windows 7 as well in a multi boot environment, but will not be using it as my primary OS before it reaches a final release.
At the time I assembled my new i7 based system (about 4 months ago) the cost of an OEM version of XP was the same as Vista. I chose vista because the 64bit version had much better driver support than the 64bit version of XP and also would have better such support moving into the future. After 4 months usage or so I am happy that I made the correct decision for my personal situation and usage.