If you aren't planning to pay for support for RHEL (after verifying that it is suitable for your intended use) don't waste your time installing it.
An unlicensed copy of RHEL won't have access to repositories for updates and adding packages.
Do you really want a server distribution of Linux? If so, and assuming you won't pay for support, get Centos instead. I expect your internet access makes downloading a Centos DVD image difficult and you already have RHEL. But despite that, using RHEL unlicensed will ultimately waste your time.
A non server, more beginner friendly Linux on CD (not DVD) is probably a better idea. Ubuntu is probably best.
Originally Posted by Deepon
kept 144Gb of unallocated space
But i don't know how to manually allocate & partition the swap, and how much to mount under '/' & under'/boot'.. And also how to set it up for the dual boot.
Forget about /boot. It serves no purpose in a simple install.
Whatever distribution you choose, the mechanics of manual partitioning should be pretty simple once you try it. For some distributions there is also decent online documentation of that step.
As for which partitions you need and how big, the defaults in most distributions are pretty stupid. You would be better off with:
2GB or 3GB for swap
The rest for /
You don't need /boot or /home or any of the other such partitions. All of those areas work best as directories within / which is what you get automatically if you don't attach partitions to those mount points.
Ubuntu is quite good at detecting Windows and setting up the dual boot for you.
Other Linux distributions are also pretty good at that.
Find out whether the distribution you will use has grub or grub2. Then find a decent online description of dual booting for that (it is quite different between grub and grub2).
Probably the Linux installer will just get everything right and once Linux is installed the dual boot will simply work. In case something goes wrong, after installing Linux, just Linux is bootable and Windows isn't. Then the online instructions are pretty easy for taking control of the dual boot setup yourself, as long as you don't make the common mistake of trying to use grub online instructions when your installer used grub2.