Kewl, well yeah, motherboards like that I do expect to be a bit pricey, but I know there around, and so I'm sure we can help.
4 gigs is usually the norm in computers, with 4 DIMM slots usually available for RAM. As for the FX 55, you'll just need a board that will support AMD pin 939 chips which an Athlon FX-55 is (you do know this costs around 500 right?! for this chip?
PCI-E is also easier to find now, with more companies branching into it, and so we can find one of these for you too, plus I guess when you say other room for a video card, your meaning to look into SLI when gaming is concerned to get higher frame rates (thats what its all about). Luckily there are alot of cards for this too, with Geforce already branching in with theres, and some ATI having some cards for them too.
ATI if you like them best are also branching into SLI, or well, not SLI, but something just like it. Check the thread on 'CrossFire', the ATI brand for SLI. (Secondary Link Interface)
As its your first project, i'll let you in on a little fact... building is never simple... but thats what makes it good, its challenging! Especially for a first time builder. I build this system myself after the other 2 computers were built by one of my dads mates. after watching him build and learning from inserting components into the old PC, I soon realised I could do it.
Luckily I didn't encounter any big problems, and I'm sure if your careful, you won't either, though sometimes, this isn't always a way to be sure. Computer have a way of turning there ugly heads as it were
Don't let this put you off...on the contary, I want it to make you enlighted, to want to carry it out.
We'll find those parts for you no problems, and we'll be here with you every step of the way if anything goes wrong. What more can you ask?
I hope this helps