All looks good.
I would definitely go for the GX2 card - you're getting SLI in one slot!!
Nvidia are working on a quad SLI driver - so in the future you could add another GX2 for (even more) amazing performance. The current quad driver is rubbish and actually results in lower fps than using normal SLI - but by the time you are in a position to go down the quad route I have no doubt Nvidai will have sorted the drivers out.
I like your choice of CPU - no point going for the higher spec when you could overclock to the same speeds with the one you have chosen.
I'm not a big fan of Sony TFT's. They generally have a shiny coating that I find reflects far too much, and I've never seen a Sony TFT come top in any comparative reviews for gaming. I prefer a matte finish, Viewsonic generally get favourable reviews for gaming TFT's, however I would do a bit of research on top line gaming screens.
My main issue with 19" TFT's is that they generally only go up to 1280x1024 resolution - your set up will easily do resolutions far in excess of this and personally I would want to be using 1600x1200 as a minimum for almost any current game on full detail. Seems kinda pointless to have all the graphical horsepower but only able to go up to 1280 resolution.
One other note - the Creative X-Fi is a great card, phenomenal infact - but I don't think an OEM version will come with an optical out - if that's no problem for you (for example if you are only connection the PC through the T7900's) that's fine - but I have a similar (older) set up using a Creative Audigy 2 Zs and T7700 speakers.
I found that it was a pain in the backside connecting up a PC, XBox360 and DVD player through the speakers. I ended up buying a Creative DDS-100 decoder so I could run all the devices through this box, and then in to the speakers. But because the soundcard has no optical output I had to use this really clunky manual passthrough switchbox that comes with the DDS. I found it annoying and eventually discarded the soundcard and used the lower quality onboard sound as it provided an optical out.
In regards to your memory speed quaestion - you'll probably not notice any difference, but as always... if you can afford it then go for it!