If you want a card, get a 7900GS, mine is just insane, nothing faster for $120. I lucked on ebay, but there are some good deals out there, don't pay more than $150.
Sli can be turned on/off on the fly, I have 2 cards on my rig and I can watch 3 720p streams full screen on 3 monitors (WMV HD) and this on a single core socket 939 4000+ @ 3ghz. SLI is useless, get an 8600GT with Pure-Video if you don't game, it is dropping in price, don't spend more than $150 for it, and get it HDCP certified (I think nVidia has promised all of them will be).
If you really want to game, ditch the processor you chose and get a single core, then spend the difference for the 8800GTS 320MB video card, it will go faster than quad SLI 7900 series, and SLi isn't perfectly support by everything as if it was a single card btw.
The current state-of-the-art is Core2 tech, see my thread for the release of a Core2 Single core for $40.
I just went to AMD dual-core, to my instant regret. I didn't choose a 3Ghz one, and my experience with my 3Ghz 939 leads me to believe you wouldn't be disappointed. The Dual cores don't clock, so my 2.4Ghz AM2 is slower than the 939 single-core was.
For a motherboard, I don't know if you have special needs, but I would get this one (and it is the PC I am using right now):
http://www.newegg.com/Product/Product.aspx...N82E16813136021Don't spend money for the other board if you don't absolutely need a feature that it offers, the board I show has massive overclocking potential, is rock-solid, and SLI or dual card compatible.
The Celeron D is a sucky processor, you need to hit 3Ghz just to reach the level of a stock clock AMD.
On the other hand, a Celeron 4xx series is a Core2 single core, I would recommend it and a 650i Nforce board heartily, they overclock just like a Core2 and are $40-65. If I was building a new PC I would get these if I was going Intel for Gaming/HTPC:
http://www.newegg.com/Product/Product.aspx...N82E16820161676 Ram $35 2x 512mb DDR2
http://www.newegg.com/Product/Product.aspx...N82E16820161172 (or $77 for a 2x 1GB kit, recommended)
Any one of these processors:
http://www.newegg.com/Product/ProductList....mp;name=CeleronOr a Core2 Duo for $120, if you feel the need for that sort of thing.
http://www.newegg.com/Product/ProductList....amp;x=0&y=0 Motherboard, open box MSI for $65, or new EVGA for $100, it is up to you, or any of these are really really good.
http://www.newegg.com/Product/ProductList....GeForce+8800GTS Pick a video card, $290-315, the 8800GTS 320MB is blazing fast for the money (2x or more the speed of the 7900GS), well worth using a cheaper or single core to afford if you game. The 8
600GTS is slower than the 7900GS for the same price, BUT if you want to have full Purevideo HD-DVD decoding it is the way to go. AND it is around $120-150 range.
The fact is that single cores are easier to overclock because of less heat, and most games aren't multi-core supporting, so the money spent on the geforce 8800 will make your system at least 50-70% faster than the one you layed out, if you are gaming at all, or doing single-threaded apps.
Steer clear of the 7800 sereis, the 7900GS is almost the same card, but faster, makes less heat, and uses less electricity. Also with proper cooling it can overclock to 650-700mhz on the core (water or Zalman heat-pipe recommended at that level). Mostly it is about the same price or cheaper, and has less noisy annoying fans on the whole.
Just some thoughts, I wouldn't go AMD on a new build right now (well, I did, and regret it), the Core2 is just faster for mainstream (apparently AMD spanks them in the server market still, but that isn't us), and at $50-65 for that technology I can't say AMD is a good choice anymore for even a budget build.
Besides, if you like the Celeron 420/430/440, but need more processing power down the road, you can get a Core2 Duo or Quad and just blaze.
EDIT, I forgot to add, if you are using Windows Media Player, it is Crap, I use Media Player Classic or Video Lan Client (VLC) for all my video (google K-lite mega codec pack for Media player classic).
I notice better CPU utilization, better interface, less bulky program, starts quicker, isn't integrated with the OS and doesn't leave you open to exploits, and like I say, 3 720p streams full-screen on a $400 PC isn't anything to sneer at, Media Player Classic is what allowed me to do it (well, aside from the 3Ghz AMD processor).
You need a storage sub-system able to move the data though, play each vid from a a seperate hard drive, I used an external USB 2.0 for one of the streams.