You can attach the HDD to the same cable as the DVD drive without a problem...
About the 2 PATA HDD's connected to the onboard raid controller:
I have an MSI board as well (K7N2G-ILSR) with onboard raid. This raidcontroller only support 1 PATA HDD and 2 SATA HDD's. Although the functionality of the controller is limited, I managed to connect two PATA disks to the controller, without any BIOS mods!
Perhaps this trick works for you as well!
How I did it?
Well, just connect both HDD's to the PATA channel of your RAID controller. You'll notice that the BIOS of the RAID controller only detects the master disk. No problem.. just make it a stripe set (of 1 disk!) and boot to Windows. Install the Promise managment utility and login to the localhost. In the list on the left, you'll see a bunch of channels and a bunch of disks. If everything works fine, you'll be able to activate the slave disc from there!
Otherwhise, you might need a BIOS mod. I did such a mod on my board, and now I'm running 1x SATA (single disc stripe) and 2x PATA (two disc stripe) on the onboard controller, that has succesfully been converted to a fully featured TX2 Plus!