You don´t need a second drive at once. It also works on one HD, given it is not extremely old. When you speak of 1.2 TByte we can almost be sure that it is an actual SATA disc, isn´t it ?
But sooner or later life gets easier with a second drive. You gain speed and overview, because you don´t get lost in the question " Where are all my samples?". I recommend to put all your libraries VSL or any other to the second drive. This way it is independent of the boot disc. Now the operation system and applications are free to do what they want, while the pipeline to your samples still is free.
It is not just about xy MB/s data stream performance, what you can read in different test magazines. For me the most attractive improvement is better reaction time and silent operation
( We all know about the access delays and noise when the harddiscs move the head over the platter. zrzgg zrgg zrgg zrggg .... Especially if the disc gets older, this can drive you nuts....)
The best way out of this of course is a SSD. Zero noise operation, low temperature and extremely short access times. The biggest drawback at the moment is the cost factor. Also there still are some open points regarding lifetime, TRIM, fragmentation, alignment, operation system compatibility etc. etc.
But I have to say, that I got mine just after christmas and it worked more or less out of the box, even under Win XP.
For samples it is an expensive solution, but for the boot disc a 60GB can be enough and you can get these now for under 100 Euro. Over 120 GB they tend towards 200 Euro and everything above is still space tech..... :)
So in case you don´t want to overstretch your budget, I´d buy a SSD, clone your boot partition onto it and use the old harddisc for the samples, audio and other data mining stuff.
If you are too afraid of disc cloning, repartitioning, partition resizing, evetually AHCI reconfiguration etc.etc., take the easy way and just wait until they get cheaper.
In the time between, try to not cluster and expand your C partition for easy migration later on. Also try to streamline your program paths for the same reason. if you have many partitions on your 1.2TB disc and the VSL player gets its samples from D partition, Native instrumemts battery on E, Cubase audiofiles on F, Wavelab on G ....... it can get very complicated in the future , unless you buy an 1.2TB SSD :)
Puting everything on C is also a bad idea.
I´d put the Windows boot stuff and the programs on C, other big library, audio, video etc files on D.
When you switch to SSD in the future, it is easy to transfer the C partition to a solid state drive. Even the data paths stay the same if you keep drive letters.