1) Yes it does have room for both. it will go in the slot below the DVD RW drive
2) Power cables are already inside. If you buy a Boxed BluRay it should come with the needed SATA cable. If you buy OEM (Usually just the drive it self without a box) it will not come with a SATA cable and you have to buy one seperatly for a few bucks.
3) I have an older LG. It went out on me after about 10 months though. Got it RMAed pretty quick. No issues since. I hardly ever use mine. Mainly just to rip my BluRays so I can have backups and watch them on my phone/XBox, but there are a few brands out there. Before i even buy anything always check out the reviews of that product on newegg and amazon. You'll have an Idea of what you could be getting yourself into regardless of what someone might say how great it is.
4) The advantage of a SSD is Speed. They are anywhere from 100 to 500 depending on Brand, Speed, and Size. If your a novice at PC's it will take some time and since you have an XPS I know for a fact it didn't come with any Recovery Media (To Reinstall windows) So you either 1) have to pay for another version of windows which you don't want to do or 2) Clone the drive which pretty much make a clone of your existing hard drive and puts it on the SSD. This though is also kinda frowned upon. its best to just reinstall Windows when moving to an SSD unless you can't.
Yea my company is actually a Certified Dell Reseller. The bought a XPS 8600 because it was on sale. We bought a Samsung 840 256 gig SSD, installed windows 7 on it (It came with 8 yuck) and then make the 2TB a data drive.
Now for just the OS and some programs a 120 gig will be good enough. Just make sure you install your games to the 2TB hard drive if you go that small since some games these days take up 15-30 gigs of space. Just keep in mind SSD's are about a Buck a Gig right now on average (For the good fast ones that is) but even some of the slower ones you will still see a good performance increase over the Hard Drive.
There are a few Good programs you can use to Clone the hard drive. Some SSD Upgrade Kits (There is a difference in the upgrade kits) Will come with a CD that allows you to Clone your hard drive to your SSD. Now this will also only work if the amount of used space on the hard drive is the same or less than the size of the SSD. Then once its cloned you can then erase everything on the Hard drive, you can format it to one big drive or split it up (2Tb is pretty big but its up to you) and then you can make things like My Documents, Video, Pictures and what not go to the Hard Drive so they don't take up space on the SSD.
The process it self isn't that hard (At least to me). Up to you if you want to take on the challenge. The good thing is even if cloning doesn't work it won't affect anything on your Hard Drive.