Now I am all fat and sassy.
A few things to keep in mind. 10 - 12 Gig. is plenty of space to put into a / partition for Mint or Ubuntu. More than that you will probably never use and it just slows down your system. A smaller partition is a faster partition. I have 7 gig. of files in my / partition on a KDE install, which is bigger than a Gnome install, and I have everything but the kitchen sink on my install, (I like to play with stuff). Additional programs are what make the / grow. All your data goes into home. If you don't have data partitions, /home is what grows. Mint already comes with a more than respectable complement of programs, so you probably won't be adding much to your install for a general purpose desktop.
Your swap partition is rather large. A swap partition should not be bigger than 2X your physical RAM. The total of physical RAM plus swap should not exceed 4 Gig. on a 32 bit system. If it does, reduce the swap size accordingly to stay at 4 Gig. or less. You don't want to slow swap down when you can't use the extra space anyway. It is better to keep swap closer to the top of the partition table for speed but since you already have it where you do we will just leave it there. Just remember in the future if you need to completely redo you partitions to put the swap right after the Windows install on a primary partition.
Ok, the first thing I would suggest is to go to the url below and download and burn a live cd called Supergrub. This is handy to have around. You can boot into almost any install with it.
Download super_grub_disk_0.9711.isohttp://download.linux-live-cd.org/Super ... sgd/cdrom/
Next go to the site below and download and burn the Latest stable version of Gparted live cd iso.http://sourceforge.net/project/showfile ... _id=271779
This should give you the tools to do what you need to do and they will probably come in handy later on too.
This is what I would suggest you do. You will be using the Gparted live cd to do this:
1) I would shrink sda5 to 10 Gig.
2) I would shrink sda6 to 12 Gig.
3) Shrink your swap partition to whatever the above formula dictates, and put the space liberated into your /home on sda7.
4) Combine the liberated space and make another ext3 logical partition of 12 Gig. It will probably be called sda8.
5) Put the rest of the space you liberated into your current /home on sda7
That should not only give you space for another system but also give you more space in your /home partition that you use for storage.
Since you just installed Mint 4 you probably don't have a lot of stuff in that install to loose. I would reformat sda6.
Install your beta on sda8 all on the same partition.
Then install Mint 4 on sda6 all on the same partition.
When you install Mint 4 last it should pickup all the installs so they will all boot ok.
Then mount the single Ubuntu /home partition in a folder on each of your Mint install /home folders. When you make your installs, be sure to use the same user name to keep from having permissions problems.
As you have questions, post back.
EDIT: When you get this far we will need to become familiar with /etc/fstab and /boot/grub/menu.lst and correct your UUIDs and boot stanzas.