Is GParted very reliable? Is it as good as Partition Magic?

I've used GParted for ages and have never had a problem, using the LiveCD edition to be specific. As for comparing it to Partition Magic, I'd say GParted has the upper hand. There is a huge community behind it for support since it's open source, and it supports a multitude of filesystems. Partition Magic is also kind of old, it's last release was 5 years ago, about when GParted had it's first release, so it's safe to say GParted is more up to date. It has handled my 1TB drives with ease, performing more than 20 operations in a single reboot sometimes.

For partitioning NTFS I'd strongly recommend reading their guides. Since you're using Vista, The How-To Geek's guide is also essential reading.

GParted is a great tool ever multi-booter should have in their toolbox.

Some discussion here on GParted and Partition Magic.
I have been happily using GParted without reading these comparisons.
You may want to take a look at Parted Magic too (it is nicely packaged).

Common problem in Windows Vista / 7 Disk Management is enabled hibernation. This file can limit the resize range as you desribed. Try to disable hibernation

powercfg -h off

Also from my experience the disk management never allowed me to shrink partition by more than 50% of overall disk space.