Linux doesnt need that much HDD space, so i guess 5Gig would be more than enuff + a few hundred MB for swap space. Then again, it depends on what you're going to use it for. Currently i have an old 10Gig HDD for my Linux server. That is more than enuff space. And yes, theoratically partitioning the HDD is creating another drive. But the fact is that physically, there is only one drive.
If you have Linux installed, as well as another OS, you can have a boot loader such as Lilo, which allows you to select which OS to boot. Also, in Linux you are able to mount your Windows partition into the Linux OS. Therefore, while in Linux, you can still access files on your Windows system.
For installation, really it depends on what stage of the installation you're in. For instance, if you've created a partition and formatted the HDD, it's not wise to just leave things as they are and do it again another time. Most likely after partitioning the HDD, your MBR and the likes would already be altered. I suggest you just install the Linux and a boot loader. That way if you have no idea how use the Linux, you can just reboot and get the boot loader to boot Windows. Then again, for Redhat 8.0 (can't remember if older version works the same) you will need to go through the entire setup stage before the installation can actually begin. You would need to set partitions, system configs and the likes, before the installer actually creates the partition, formats it and installs the Linux files. So if you're in the process of setting up the installation and you're clueless of what to do, you can simply quit the installation and things will be back to normal.
The best way to do this is to read HOWTOs i guess. I STRONGLY suggest you read HOWTOs before you even think about installing Linux. All the information you need is in the HOWTOs. From installing to configuring the Linux system itself, everything is well documented. Try searching for Installation HOWTOs on Google or even the Redhat network at
Well good luck in installing Linux. It's not that scary.. really.