Wow, talk about some rude and unhelpful answers. Sure, the OP isn't asking about something you specifically care about, and his grammer and spelling isn't perfect but neither was yours.
So here is a more useful answer (hopefully):
NVIDIA 8800 GT Is the DX10 capable in windows ?
Booting into windows do you lose any of the capabilities of the card? will it still be 512?
Booting into Windows results in full access to the card. This includes full access to the video RAM, feature-set, etc. Being an nVidia 8000 series card, you will be able to run DX10 applications, but to do that you will need run Vista (not XP) which isn't ideal but will work fine.
can I buy two NVIDIA 8800gt and use both as one video car in windows?
In theory, you can run two 8800GTs but Apple don't offer this as a standard configuration. This is probably due to both heat and power considerations. The Quadro card takes two auxillary power inputs, but the 8800 only takes one so that suggests that it is technically possible however I would do more research on this first. Remember that only one of the PCI-Express slots on this board is 16x, the rest are 8x slots. On the older Mac Pro you could configure the bandwidth per slot using software, but on this one you are locked in. This means that only the first card will run at full speed. I tried to find a tech reference to confirm this, but couldn't so there's a chance I'm mistaken about the secondary slots (they may be a mix of 4x and 8x).
This in turns suggests that SLI and Crossfire configurations, while potentially do-able, aren't going to yield particularly impressive performance gains.
Also, you only have 300W of power to play with (absolute max) on the PCI-Express bus for this machine, so dual 8800s means no raid controller or any other PCI cards.
Finally on this, under OSX you won't be able to use an SLI or Crossfire configuration - there is no support for the master/slave ATI cards under OSX and no nVidia SLI support either so you would probably find yourself running nothing but Windows if you went down this path.
Is the Mac able to support two ATI Radeon HD in crossfire in windows ?
Quite probably, as Crossfire does not seem to be particularly heavily dependent on BIOS or chipset. Still, going back to PCI slot bandwidth and OSX support for the cards, this isn't an ideal config.
The four PCI-E buses are they x16t or is x16 shared between the four?
Answered above (I think).
You want a gaming rig that also runs OSX, has some basic expansion options, and is much more powerful than anything else you've used. So a Mac Pro is a reasonably good answer. The Harpertown processors aren't specifically designed for gamers, but then again few CPUs are - games just happen to demand a LOT of processing power, and the Mac Pro meets that requirement. Multi-threaded games are going to have far more processing grunt than they can reasonably expect to use.
The video card options aren't too bad either - the 8800GT isn't quite at the level of the 8800 Ultra, but it is close. It will, however, age and Apple have a history of never upgrading their video cards (old Mac Pro owners will agree with this, and are still sobbing over their inability to use the new 8800GT card on their 6 month old workstations). So if you want to run OSX on your machine, bear in mind that video card upgrades are not likely to be easy and may not even be possible.
Mac Pro RAM isn't cheap, they went for server-grade memory which means it costs an arm an a leg. Still, OWC will sell you 4GB for $200 USD these days so building a machine with 6GB isn't totally out of the question. And yes, games use a LOT of memory but under Windows won't use more than 4GB (actually, a bit less than this but who cares for the sake of this conversation). If you plan on using VMWare Fusion or Parallels Desktop under OSX, I'd strongly advise 6GB of RAM. If you plan on doing extremely heavy video or graphic manipulation, 3D rendering, or serious CAD, the rule of thumb seems to be 1GB RAM per CPU core (so 8GB). Fortunately there are 8 slots, so you can upgrade in stages if you like.
Hard drives in the Mac Pro are easily replaced, and are one area where they really did great work. And the box is quiet, looks cool, is reliable, and comes with a sexy operating system on it.
This model of Mac Pro is very new, so don't expect to find a whole lot of definitive information out there - with a 3 week+ shipping delay on 8800GT equiped units, very few people have their hands on one yet.
Personally, I've been playing Windows-based games since PCs came into existence. My Mac Pro is on order, and will replace my Core 2 E6600 based workstation (4GB RAM, 8800GTS, 2TB disk, dual LCD, etc). I've ordered an extra 4GB of RAM and a couple of 1TB hard disks (from a third-party, Apple wanted my firstborn for them) and opted for the 8800GT. In my view, for day-to-day use it won't be noticably faster but for gaming it will scream. I'm going to run mine in OSX most of the time and use VMWare Fusion to retain access to my Windows applications. Gaming will require me to bootcamp into XP (so no DX10), but that is bearable. Sure, it'll be an expensive machine. But I see it outlasting any machine on the market right now.
Hopefully all this help.
Oh, and guys? Once in a while try helping the guy out instead of mocking him for wanting to do something you don't agree with? bryphotoguy, you come across as a real bully in your posts - perhaps it isn't intentional, or perhaps you just are one. Either way, wind it back eh?