motherboards
ASRock 4Core1600P35-WiFi+ LGA 775 Intel P35 ATX Intel Motherboard - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157121
$129.99
Supports DDR & DDR3, Wi-Fi and Firewire
GIGABYTE GA-EP35-DS3L LGA 775 Intel P35 ATX Intel Motherboard - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128337
$89.99
Good board for overclocking, great value no firewire, no wi-fi, DDR2 only...you only need DDR2 anyway
ASUS P5Q Pro LGA 775 Intel P45 ATX Intel Motherboard - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131299
$149.99
P45 motherboard which is intel's newest chipset, PCI Express 2.0, 65nm chipset instead of 90nm of P35, firewire
CPU
I would get the e8400 and then upgrade to a quadcore in the future..lets say a year from now when more games and applications can take advantage of all 4 cores and prices have dropped
RAM
CORSAIR XMS2 DHX 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145194
$109.00(79.99 after mail in rebate)
This ram has 4-4-4-12 timings which is about as good as it gets..and they can be overclocked to around 1Ghz if you massively overclock your CPU
OS
You will need Vista 64 bit system to use 8 gigs of ram
Video Card
Since you are on a budget you should get the ATI(AMD) 4870 GDDR5 when its released the last week of June
Power Supply
CORSAIR CMPSU-620HX 620W ATX12V v2.2 and EPS12V 2.91 Power Supply 100 - 240 V UL, CUL, CE, CB, FCC Class B, TUV, CCC, C-tick - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139002
$164.99(139.99 after mail in rebate)84% efficiency, 3 12V rails 50amps combined
CORSAIR CMPSU-520HX 520W ATX12V v2.2 and EPS12V 2.91 Power Supply 100 - 240 V UL, CUL, CE, CB, FCC Class B, TUV, CCC, C-tick - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139001
$119.99(99.99 after mail in rebate)84% efficiency, 3 12V rails 40amps combined
The case is going to be up to you because some people just pick them on looks alone