I'm building a new PC for gaming and have a budget of $800
 
This is what I have so far:
 
DVD-RW: Pioneer IDE (I already own)
 
Case: AeroCool ExtremeEngine 3T (Already own)
 
OS: Windows XP Professional (Already own)
 
HDD: Seagate Barracuda 7200.10 500GB SATA 3.0Gb/s(Already own)
 
Mobo: http://www.newegg.com/Product/Prod [...] 6813128078
 
CPU: http://www.newegg.com/Product/Prod [...] 6819115037
 
GPU: http://www.newegg.com/Product/Prod [...] 6814133217
 
PSU: http://www.newegg.com/Product/Prod [...] 6817341001
 
RAM: http://www.newegg.com/Product/Prod [...] 6820227198
 
Price after all rebates: ~ $730-$740
 
The fact that I own some of this already makes it easier, and I don't mind upgrading later, just right now I don't have enough cash for a new case, HDD, etc.
 
I don't think the E8400 will bottleneck the 9800 GX2, but I think the fact that the mobo is PCI-e 1.0 will, but you can't get a PCI-e 2.0 mobo cheap enough.  
 
Any input appreciated.

None of the new video cards fully support the PCI-e 2.0 standard anyway so thats not a problem.  
 
The only problem you may come up with is the motherboard and support for your choice of cpu. The board does support it but only after a set BIOS file. If you get a board that has been setting around on a shelf for some time it may have an older version of the BIOS and may require an updated BIOS to support your cpu. There is no real way of telling tell you have the board in hand.

I'd replace the GA-G31M-S2L ($68) with a GA-EP45-DS3R ($158)
http://www.newegg.com/Product/Prod [...] -EP45-DS3R

and the GameXStream 600W ($94) with a Corsair 750TX  ($120)
http://www.newegg.com/Product/Prod [...] &Tpk=750tx

This adds $116 to your bill, but your GX2 will get the PCI-E 2 slot it needs and enough power to run properly.

Edit: a GA-EP43-DS3L at $108 would also do.
http://www.newegg.com/Product/Prod [...] pk=GA-EP43

you didn't mention a screen resolution.
 
a GX2 is esentialy just an SLI pair.
 
at lower resolutions, you might be better of with a good single card.

^Agreed.

I agree with aevm about the MB.  Also agree with groo.
Mention what screen size/resolution you're going to be using (now or in the near future).
You can "size" the video card and power supply to the monitor resolution.
1920x1200 = 9800GX2 = 600W PSU
1600x1200 (or lower) = 9800GTX = 450W
(power reqmts from the Evga Product list specs @ http://www.evga.com/products/prodlist.asp )
 
I know this is a HD 4850 review but it also can give you an idea about the 9800GX2 and 9800GTX performance comparisons at 19x12 and 16x12 resolutions.  http://anandtech.com/video/showdoc.aspx?i=3338&p=6
 
and 9800GTX now selling @ $200 http://www.newegg.com/Product/Prod [...] 6814133219

^ P43/P45 mobo

Yeah, you have picked out some really nice top end components....why are you going to skimp on the MOST important part of the build, and put them all on that small, (just try to fit a 9800GX2 onto this board, Ha-Ha....) featureless, old, cheap-ass motherboard?????

Well I inherited some $$$$ so I am going to start from scratch. Reposted as a new thread, using more cash.

nothing like blowing money as soon as it falls into your hands.