Allright i have a e6750 + hd3870 + 2 gb ram + x-fi sound card. Now all i need is the other parts to complete the build. Last time i got parts without consulting the tomshardware experts(u, u and that guy over there) it was no good. I want to do xfire when the hd4870 comes out. So i need a mobos, psu (im thinking cosair 650,) and a casing and also stuff i didnt/forgot to mention. I wanna spend between $300 to $500 on this, lower being better. Thanx

Ok thats a good starting point. Xfire means you will want an intel chipset motherboard so I recommend looking at the X38, P35 or the new X48 chipsets from the likes of Asus or Gigabyte. P35 will probably give you the best value for money but if it was me I would go with the newest, the X48 probably because I just like to be up to date and it will last you a long time.  
 
PSU - Yes corsair PSU's are good, but if you want to run xfire in the future you may want a bit more juice. For example, a current 3870 requires a 450 watt minimum and so running xfire will need more. If you want to stick with corsair then maybe go with the 750w if not have a look at this list http://www.xtremesystems.org/forum [...] p?t=108088 and choose a brand as close to the top as possible.
 
Case I can't really help you with as it really is down to personal opinion and how you like your PC's to look. Good brands are Antec, Lian Li, Thermaltake among others. Choose one you like and give me a link to check it out before you do anything. Its important that you don't spend too little on a case otherwise you will end up with a noisy, hot and of poor build quality case.
 
Other stuff you may want to consider: Hard drive and possibly an aftermarket heatsink for overclocking.

P35 will not do Crossfire (Xfire is a communication program )....it has only one X16 slot.  The X38/48 will.  Now between X38 and X48 its hard to say.  The X48 gives almost no difference but has slightly better support for a 1600FSB.  If you have the money get the 48 otherwise get a good X38 (Maximus Formula (not extreme))

i say for a case if you want to keep it cool, go for the Antec 900 or the new, slightly expensive Coolermaster Cosmos S.  I love the 900, definitely a great air-cooled solution.

The 650 will handle 20 3870s without a problem.  
 
I agree with nzxt on X38 or 48 if you are serious about xfire. The P35 boards will do crossfire but are hindered by the restrictions of the second PEG slot (4x). There is a big price gap there though (about $100)  
 
As far as cases go, I like simplicity, so something from coolermaster in the $50 range would be good.
 
Depending on your needs, you can get a 750GB HDD for around $100

 
 
You're wrong, many many P35 boards WILL do crossfire.
 
http://www.newegg.com/Product/Prod [...] 6813131180
 
http://www.newegg.com/Product/Prod [...] 6813128083
 
and many many others

AMD Certified PSUs (Single+Crossfire):
 
http://ati.amd.com/online/certifiedpsu/index.html

Sorry about that...I wasnt very clear.  I know the P35 can do Crossfire but to me....a 4x slot isnt REALLY Crossfire. Its like putting 2 8600s in SLI.  Its almost worthless

alright i guess im heading into x38 area. Wow psus are so expensive i love the corsair company but its not tier 1. For $200 i might aswell get something like enermax galaxy or antec. 200ish for x38 + 200ish for psu + 150ish for HDD + 300ish for 4870 Im having second thoughts of even doing xfire... http://www.newegg.com/Product/Prod [...] 6813131188
wat bout this board doesnt it have 2 pci-e 16. Thanx for the help

^ I recommend you stay away from those DDR3 + DDR2 combo boards. They seems to have quite a bit of compatibility issues. Just get a Normal X38 board with DDR2.

---------------
E2180 @3.2Ghz + P35DS3L +8400GS (700/475 OC)  

^^ Agreed
 
Also, those Corsairs can be considered Tier 1 PSUs.  They are very good, dont base all assumptions on that list.  Antec on the other hand......only stick with the quattros and the Enermax Galaxies

 
I guess so, but I just don't think X38 is worth $100 more then a good P35 board. Having crossfire as option is nice, but I am a fan of a good single card.

 
i agree with you completely with being a fan of a good single card, but if i get the 4870 and i have a xfire-able 3870 lying around, why not xfire?