There is no reason not to go with Vista in my opinion.
I'm not familiar with the i7 chip range, but I would hazzard a guess that you will need a 64-bit OS. (If you're going with >= 4GB RAM, then you will need 64-Bit anyway).
Vista Home Premium is the version to buy. Seeing as you're going with an i7, you won't be upgrading your motherboard any time soon, so you could go with OEM, however, I would still go with Retail.
Also, if you wanted to stay with XP Pro, it depends if your licence is OEM or Retail - if it's OEM, then you will need to buy a new OS, in which case, get Vista.
Hope all this helps.