I'm looking at building a new computer, and I have everything picked out, except for the MOBO, and video card. I'm hung up on wether or not to go with 2 ATI X850's running crossfire, or to get a X1800xt when it comes out. The computer will be a gaming, video and audio editing computer. Any suggestions?

2 x850 is the "SLI"?
i think 550xt and nvidia 6500 is the low price graphic card and the one you were going to buy is the high lelve.
and i have reading some test article,the SLI maybe not so better than single one,(if the price of two is the same)
and if you runs SLI,there are not so many games support it.

The crossfire setup is not like SLI. From what I've read, crossfire seperates more that one section of the screen, unlike SLI that divides it in half. Also, there is no such card is the X550. It is not a mater of games not being able to run SLI or Crossfire, it is the OS beiong able to support it, which just about all do.

I also don't which graphics card I should get, I have an intel extreme and it can't run anything. I'd want one good enought to run Half Life 2 (probably the most gfx intense game I'm gonna get) but it has to be under 250$ (Christmas sales are coming up so it'll probably be cheaper by then). I want an ATI, but the problem is that there are so many I don't know which one is best for me...

I've been reading up a lot on the midrange graphics cards, and it seems that the best card you can possibly get for around $150 is either the 6800, or 6600gt. It really depends what you are going to use it for, and the resolution you usually play at.
If you like to use antialising and antistropic filtering, go for the 6800 because it has more shaders/ pipelines ect. Some can also be unlocked in some cases, making perferm closer to that of a 6800gt. However, if you usually play on lower resolution settings, such as 1024x768, then you should probably go for the 6600gt with its faster gddr3 memory. I've also heard that these can be OC'd 10-15% to get about 5-10 more fps in most games.
Either of tese should be fine for playing anygame that is out right now or any that will come out soon. Especially HL2 and source. Both of these cards should run near 100fps even with slight AA and filtering on. Plus theres a lot of sales and rebates going on at newegg and zzf, you can probably snag one of these for under $130 if you look hard enough.

ATI is slow with its releases as you may have noticed and Crossfire doesn't work or is supported as well as Nvidia's SLI. So if you want a dual video card setup I would definately recommned you an nVidia SLI setup. The new beta drivers from nVidia also increase SLI speeds and options. You can now enable and disable SLI without a restart and mix cards. For example you can have a 7800GT and 1 6800 SLId together.

However if you want to go with a single card solution and have the money, you can get an 1800XL which are out now or the 7800GTX with 512MB of memory. Both cards are extremely fast and to be honest you wont notice much difference in game speed unless you play at insanely high resolutions with AA or AF on.