To show how things look now on the 640gb and how you can reshape the partitioning compare the three images made in paint to the first of the two images early of the disk management to see how the drives looked when split for dual boots there.
Unfortunately you can't write things out in paint so you'll have to excuse the penmanship. The "V" subs for Vista while the "7" is obvious with the first showing how the drive looks now.
The second shows how the 7 primary is moved towards the front of the drive once the Vista primary is reduced in size. The third space in total black is now raw empty drive space with nothing on it.
Now to do what is considered merging partitions the 7 primary once having no free space left inbetween that and the first Vista primary you simply expand the 16gb 7 primary into the empty drive space at the back end of the drive now seen to come out with the third image.
On a 640gb drive depending on how much free space is available on the Vista primary you'll want to leave at least a good 80gb free to prevent running out of room there as well as any new files added onto it. The new look for the 7 primary can easily see 100gb or more!
Unless you have tons of files on the Vista primary you'll want the new 7 theme to provide far more drive space and eventually you won't even want Vista on much longer once you get accustomed to 7! Here after a nice clean install of Vista with SP2 on I created an image to tuck away until the 32bit RC has had it's day with Vista having been the former resident OS there as you can see from the first image earlier.
And for dual booting two versions on two drives each should see all programs installed on it's own primary. That frees up the entire second drive for the next OS which just happens to be 7. The best place for a dual like this would be seeing 7 on the first partition of the second drive even if the drive is split up. The second would simply be a storage/backup partition in the event you end up reformatting either of the other two(Vista or 7 for any reason).
For getting familiar with booting from a live cd version of GParted or the full name Gnome Partition Editor review the information seen at GPARTED DOCUMENTATION - GENERALITIES
Some extra screens are seen at GParted -- Screenshots
For burning a live cd you will want the ISO not zip file download of the latest 0.4.5-2 release available at SourceForge.net: GParted: Files
First look over the first partition to see how much drive space is used. Once you know the percentage you can go by that to leave plenty of space while still seeing that one shrunk down. Moving a partition back however does take some time. But first read through the documentation.