There are definitely 2 schools of thought on a dedicated partition for /home. As Fedorarefugee suggested, you could definitely include the /home into the main ops partition (that's the other school of thought
The poster indicated that he wants to expand into other distro's / versions at some point in the future. For that reason, I recommended using GParted and create the extended partition up front ( I don't think you can create extended as part of the installer partitioning capabilities??). If he goes with his 4th partition as a data partition in ext3, he's going to be faced with a significant challenge down the road if/when he wants to expand to another partition(s) for the additional ops.
Just a bit of my rationale for whatever it's worth...
Not if he includes /home in root. Then he will have two root partitions (for two distros / versions) a shared swap and a shared data partition for his personal stuff. If he wanted separate /homes then he could extend both root partitions and include /home and /whatever else keeping the distros partitions all together (though this is irrelevant except making the partition table easy to understand). If he wanted to boot three or more distros then he will have some head scratching to do. In any case, I especially do not recommend sharing a /home between two distros, or even two versions of the same distro. It can be done but it will lead to many problems. Easier to just keep your personal stuff on a separate partition. I have had the same /data on my main desktop since FC4, which was about 4(?) years ago now. It has seen upwards of ten different installs by this point, I am currently running Shane's Fluxbox 6 version, and have never had a problem mounting and reading/writing to it.
I believe you are correct about not being able to do extended partitions in the Mint installer, at least that I have ever noticed. In this case it would probably be best to set the table up with GParted. The only reason I suggested otherwise when using primary partitions is because when you create them with the Mint installer you will be sure to format and/or mount them.
edit: Also, in response to Aging Technogeek, and in reference to smaller partitions, I agree. Especially if you are dual booting two distros and do not intend to install much beyond the default apps then by all means keep them as small as comfortable. A standard ISO install can usually be done in ~7GB even with /home included. All the more reason to keep personal data on a larger, inside partition. The only reason I suggested 20GB for / was if you only ran the one distro and intended to leave it installed and wanted to feel free to install everything in sight. On a 160GB drive 20GB is not really cutting into anything and this is a good size for total comfort. If you will not need it you should not waste the space though.