No, you don't want to use multisite for that. Multisite is still a lot of extra work to setup and maintain, you don't want to bother with it unless you have many different sites you want to power, or a blog network.
If it is part of your site, on the same domain, I'd keep it in the same WordPress install. Then you don't have to kill yourself updating two sites and keeping both the themes up-to-date and matching. Not to mention you might want to be able to use the content from one section in the other, like a latest posts widget.
There are a few ways you could approach this depending on how much you want to customize your theme.
The most basic way would be to just create a portfolio category, exclude this category from your main blog, and create a custom category or page template to create your portfolio page. This is what I do on my own site.
The only things to watch out for with this approach is that you will probably want to customize the single post view differently for the blog and portfolio posts.
The other thing is that all the blog and portfolio posts show up in the main feed. I actually use this feed in some places, but for my blog I have a feed for just that section so that subscribers aren't getting portfolio posts in their feed reader.
Another option, if you want to completely separate the portfolio, you could create a custom post type. That would be a nice clean beak, but you might have to mess with your theme more to integrate that custom post type. There is also a handy plugin for custom post types.