850W? Jumping jesus on a pogo stick! With that system, it's unlikely you'll even need HALF that wattage! For fark's sake, that's $450!! Just grab a 500-600W PSU. There's a reason I always recommend the Antec 550W TPII--it's cheap, it's reliable, and it's powerful enough to power damn near anything you can imagine putting in your system. Get one of those--with the money you save, you can buy another 3800+ for me!
I agree with Bluefinger, XP Pro is a good choice for dual-core, and you can find it for about $150 OEM on NewEgg. Consider if you REALLY need that 12-in-1 drive--most people find them about as useful as a floppy these days.
You can probably take all your parts down to your local computer accessories place and have them assemble it for you. (I'd avoid the big retailers like Best Buy; if they even do put together parts you bring in, they'll charge way too much. Go locally-owned if possible.)
But seriously, putting a computer together isn't that hard. I'm a freaking English major for god's sake, and I put my first one together two months ago with no irresolvable problems. Just do some more online research and read your mobo's instructions closely before putting stuff together, and I'm sure you'll have no problem. Don't be afraid to give it a shot--you'll be so satisfied after putting it together, turning it on, and customizing everything ALL BY YOURSELF. And then you'll want to do it again!
Man putting all the stuff in the box is the easiest part; just make sure you are in constant contact with the case to avoid static sparks that could fry your system.
It’s all just putting a round peg in a round hole, a square peg in a square hole. Read your mobo manual it should explain everything about how to install everything into it. (well mine did, in full detail step by step, for cpu vid card ram power supply hell every connector/slot on the d@mn thing)
also if you want to learn how to put together a pc tear an old one down and put it back together, your new one will have different slots and stuff but it will give you an idea of how hard to push the ram and vid card into place. If you take your time and just look at stuff its simple enough to figure out I did it so can you and if you need more help just come back somebody will help you out if you get stuck or are uncertain about something sh!t Ill help you if you want.
And yes it appears that all your parts will work together.
XFX PV-T71F-YDF9 GeForce 7900 GTX (665MHz) 512MB GDDR3 PCI Express x16 Video Card - Retail $530
AMD Athlon 64 X2 4200+ 1GHz HT Socket 939 Dual Core Processor - Retail $357
ASUS A8N-SLI Premium Socket 939 NVIDIA nForce4 SLI ATX AMD Motherboard - Retail $159
Crucial Technology 2GB (2 x 1GB) 184-Pin DDR SDRAM DDR 400 (PC 3200) Unbuffered Dual Channel Kit System Memory Model CT2KIT12864Z40B - OEM $175
Seagate Barracuda 7200.8 250GB 3.5" Serial ATA150 Hard Drive - OEM $95
Antec LifeStyle SONATA II Piano Black Computer Case - Retail $100
NEC 16X DVDñR DVD Burner Black IDE/ATAPI Model ND-3550A - OEM $40
Microsoft Windows XP Professional With SP2 - OEM $140
D-Link DWL-G510 IEEE 802.11b/g 32bit PCI2.2 2.4G Wireless Adapter up to 54Mbps Data Rates 64/128-Bit WEP, 802.1x, WPA - Retail $36
the total is $1632, all ordered from newegg.com you save about $600 and get better parts with this build. you could even go dual video cards or a better processor but if i was you i would put the money you save aside for future upgrades maybe next year when DX10 is starting to be used