Do a speed test on your wireless computer click here
Try it at two or three different times of the day and post the results.
Just wanting to see if the "B" router is giving you the full bandwidth of 2Mb.
As to the vista bit, if the router is giving you access to 2Mb bandwidth, they would be no need to upgrade your router.
there is no reason I can think of to change a router, just because you are getting a different operating system. the only issue I can think of is if the "b" standard router is giving you the maximum connection speed.