First Microsoft has not confirmed anything. By some leaked slides it is expected to be released on May5 not April.
If I were You I would not waste money on Vista and try W7 RC when it comes out. Or You can download leaked from torrents now.
I would not get XP x64 under any circumstances becasue of poor support. If You have copy of 32 bit you might use that until W7 comes out.
If You do not own any OS then try W7. If that does not work out buy XP32.
I am running W7 beta on two laptops and everyting is fine, better then Vista or XP. I am looking to switch to W7 RC when it is released on my main desktop PC which is currently running Vista x64.