Intel has added RAID-0 TRIM support so far for their 7 series chipsets on s1155 (such as Z77) and are currently working on adding RAID-0 TRIM support to X79
if you're going to RAID-0 the drives I'd keep them on similar ports, either both SATA6 or both SATA3. If you run them on SATA3 the drives will be held back by the individual speeds of SATA3 although that will then be "doubled" via RAID0 so it should act a lot like a single 512GB SATA6 drive.
That being said, I think it would be a shame to waste the potential of such excellent drives by neutering them with SATA3.
If I were in your shoes (of which my own situation is already very similar) I'd just RAID0 the two 830 drives on the two SATA6 ports and install all OS/apps on that array and just forget about trying to juggle software across two different "drives" with the 120GB drive (move it to a backup rig, sell it, use it as a scratch drive, etc)
My own situation is similar, as I mentioned before, in that I already run a 2 x 256GB Samsung 830 setup. And in my case I am compromising because my Steam collection alone is well over 500GB (I think I'm approaching the 700GB mark), so I simply uninstall the games I don't intend to replay all that often (the large single player games without much replay value are the first to go)
Right now I'm just trying to "survive" with my current method until SSD cost/capacity is down to $0.50 per GB and would then upgrade to a 2 x 512GB setup, of which TRIM should be available for X79 by then, although my Steam collection very well might push the limits of a 1TB setup in that same period, who knows