I looked on Sony's web page. The specs for your machine make no mention of a Firewire port, but the pictures show something that looks very much like one on the side of the case! And S400 is just another name for Firewire 400.
So, if it IS there, a simple 4-6 pin Firewire cable will work with your Fireface 800. Connect it to the Firewire 400 port on the FF of course! RME tell us that Firewire 800 is only required when daisy-chaining two FF800 units. Us mortals can get along perfectly well with Firewire 400 (and I think most of us do:-)
The cable you mention is the correct type, but rather over-priced.
You might also like to run DPC Latency Checker on your laptop. Grab it from
http://www.thesycon.de/eng/home.shtml
Unfortunately, many laptops (particularly those crammed full of "features") suffer from latency spikes making ANY serious audio use problematic. Maybe yours does, maybe not. You may as well find out now if you're likely to have problems. And I'm afraid if you do, adding a FURTHER device - a separate Firewire card - is no guarantee of a cure.
The FF800 has a LOT of in/outs. Probably more than a laptop should realistically be expected to deal with. Do you really need all those channels, the ADAT ports, etc? Maybe the FF 400 would suffice. Maybe what you really want is something with fewe advanced features but more mic inputs? You're using a laptop, so you need portability. What do you DO when you take it out? Multi-mic location recording?