should i still partition my hard drive for the OS and Data (separate), or let it run free?

Over the years I've not in general been a fan of multi-partitioning one's boot drive except in some unusual (read "rare") circumstances. By & large we've counseled users to set up their boot drives as a single partition (other than the creation of System Recovery & similar small-sized partitions that may be mandated by the OS and/or OEM) and organize their programs/data through the creation of folders (rather than separate partitions).

While I'm still inclined to feel that way I'm not quite so adamant about it as I once was. And that is chiefly because of the advent of SSDs over the past few years as more & more users (wisely) install that type of disk as their boot drive because of the enormous speed advantages that device yields as compared with a HDD.

So what's the problem here as it affects the organization of one's boot drive? Because of the substantially higher cost of SSDs as compared with HDDs on a per/GB basis, most users cannot afford the larger size SSD that's needed to accommodate the total data of their current HDD-based system when the time comes when the user decides to use a SSD as his/her boot drive. They are unable to clone ("disk migrate") the contents of their HDD source disk to a SSD destination disk because of the disk-size differentials. There is no practical way of isolating the OS integrated in their single-partitioned source HDD and cloning only the OS over to the smaller sized SSD. On the other hand had they originally set up their HDD so as to create a partition solely to contain the OS and another partition to contain the remainder of their data contents there would be no problem. The OS could then be cloned over to the destination SSD so that it would serve as the new boot drive with the HDD now serving as a secondary drive in the system.

So ordinarily I would tend to agree with slingsrat's suggestion about setting up your 750 GB HDD with a single partition to contain the OS + all other data. But these days I'm not at all certain that's the way to go in view of what I've previously mentioned. Obviously it's possible you have no intention now or in the future, nor can you forsee such, where you may want to utilize a SSD as the boot drive for this system. While a 1 TB SSD would accommodate the total contents of the present HDD the steep cost of that large-sized SSD might prohibit its purchase. On the other hand, a more economical 120 GB or 240 GB SSD would at least accommodate the OS if your source HDD contained a partition solely devoted to the OS.

So the possible bottom line here is perhaps your original idea of creating a 100 GB partition to contain the OS is a sensible precaution. I cannot envision that by doing so it would adversely affect the current performance of the system in any significant way.