Have to decide between AMD and Intel. I'd go AMD. Probably don't need an X2 but the prices have come down considerably..
http://www.newegg.com/Product/ProductList.asp?Order=PRICE&Page=1&N=50001028+2010340343+1051707439+1302820275&Submit=ENE&Nty=1&Subcategory=343On the motherboard, I like Asus personally. I'd stay away from the ones that have all fufu stuff on it. Onboard audio, onboard wireless, etc.
Possible choice..
http://www.newegg.com/Product/Product.asp?Item=N82E16813131014On the case.. I like Antec, except their cheaper cases have cheap power supplies. I'd spend a little more money here. This is a great case.. If you can find it locally you can probably save on shipping.
http://www.newegg.com/Product/Product.asp?Item=N82E16811129155 On RAM, you'll want at least a gig in dual channel. I'd also double check the qual list for the motherboard you choose. I've had good luck with Kingston and if you step down into DDR2 667, you can save a few dollars.
http://www.newegg.com/Product/Product.asp?Item=N82E16820144199For your hard drive, tough choice between SATAII or 10k Raptor..
http://www.newegg.com/Product/Product.asp?Item=N82E16822135106http://www.newegg.com/Product/Product.asp?Item=N82E16822136054On the video card, the new slot is PCI-E. I've had good luck with the XFX Nvidia cards.
The cheap choice..
http://www.newegg.com/Product/Product.asp?Item=N82E16814150152or a step up..
http://www.newegg.com/Product/Product.asp?Item=N82E16814150154You're on your own on operating system.
As far as assembly, it really isn't that difficult. But if you've never done it, I'd at least try and find someone to give you a hand.
If you want prebuilt, Costco is carrying some nice HP boxes with X2 processors, but I think they come with monitors and all.
Whatever you do, don't go Dell, Lenovo or Sony.
Hope that helps.