Based on my experience (and countless reviews all over the net) the i7's are very fast. An article
in PCWorld even states...."Core i7 seems to be designed to work with Microsoft's upcoming Windows 7 operating system. He said the computer Intel sent him to try out is running Windows Vista but that chip designers had specs on Windows 7 while they were developing the new chip."
Or read this post
Bear in mind that an "upgrade" to an i7 calls for a lot of new items (from Tom's Hardware
review of the chips):
"The new CPUs will only work on expensive boards based on Intelís current high-end chipset, the X58. Also, it runs exclusively with DDR3 memory, which isnít exactly cheap either. The new LGA1366 socket also requires a new cooler."
Which chip? Depends (obviously) on your budget. I've got the 920 and am very happy with it. With the right hardware/expertise you can overclock this chip
Just a few googles and you'll have more info than you can imagine to help make up your mind.