Using XP or Vista is very much a personal choice. I went from XP to Vista about a year ago now and wouldn't even dream of going back to XP (yes, it's that much better). Like most versions of Windows (9x - Vista) you can set up Vista to look and feel like XP if you want. I did when I first installed it but now use the Vista interface as it is easier and certainly better.
You will likely want to transfer your data files from the XP machine to your new Vista machine. This is easily done using a USB Flash drive or a couple of CDs. Just copy the data files to either medium and copy them back on the new machine. There are other methods of doing this but the way I described is very easy to do.
My advice would be to bite the bullet and go for the new machine and Vista. You will be a little confused at first but it won't take you long to get used to it. Good luck with your new machine.
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Cheers.... Kev
"The Definition of an Upgrade: Take old bugs out, put new ones in."
Please don't send me PM's for support. Post in the forums for everyone to benefit.