Vista, x64 if >4gb of RAM.
I know all of you anti-Vista and anti-MS people will disagree, but here is my reasoning.
- Vista really isn't that bad and Windows 7 is even better.
- If you are going to upgrade to Windows 7 in the near future, keep in mind that an upgrade install directly from XP to & will NOT be possible. You will need to do a clean install of 7, which includes reinstalling all of your old software...
Other thoughts:
- You can install the Windows 7 Beta, then install the RTM version when released, but AFAIK MS hasn't officially stated they will support this, yet. I can't say I recommend doing this, but 7 beta does seem very stable. I know many people who have already made the switch.
- Don't try to use XP x64. I've heard nothing but bad things about it, especially with newer drivers.
- In any case make sure you you have the latest service pack (SP3 for XP, SP1 for Vista), preferably pre-installed from the install disk.