Most distros will detect Windows, resize the Windows partition (allowing you to decide exactly how much/how little to allocate to Linux) and install dual-boot all "automatically", with zero effort on your part.
I'm not familiar with Mandriva ...
... but if it doesn't automatically handle dual-boot (with little or no manual intervention on your part), then you might want to consider another distro.
Simply Mepis (http://www.mepis.org
) and SuSE Linux (http://en.opensuse.org/
) are two I've personally used (for many different versions) and vouch for. There are many others to choose from.
'Hope that helps .. PSM
1GB RAM is more than enough for any distro, 10-20GB disk should be more than enough for all of Linux - including the OS, the swap partition, and all of your Linux files. And I wouldn't worry too much about the size of your swap (whatever install recommends should be fine) ... because with 1GB, you might never even *touch* your swap space.