Bit of a tight budget with an OS and monitor but here goes.
Your two already chosen parts plus...
AMD Phenom II X3 710
http://www.newegg.com/Product/Produc...8&Tpk=X3%20710
$125
GIGABYTE GV-R455D3-512I Radeon HD 4550 512MB 64-bit GDDR3
http://www.newegg.com/Product/Produc...82E16814125250
$44.99
BIOSTAR TA790GXB A2+ AM2+/AM2 AMD 790GX
http://www.newegg.com/Product/Produc...82E16813138143
$109.99
Western Digital Caviar Black WD5001AALS 500GB 7200 RPM 32MB Cache
http://www.newegg.com/Product/Produc...82E16822136320
$74.99
Microsoft Windows Vista Home Premium SP1 64-bit for System Builders
http://www.newegg.com/Product/Produc...82E16832116488
$99
OCZ Reaper HPC Edition 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800
http://www.newegg.com/Product/Produc...82E16820227267
$28.99
ENERMAX Tomahawk ETK405AST 405W ATX12V V2.2 Power Supply
http://www.newegg.com/Product/Produc...82E16817194037
$44.99
LITE-ON Black 22X DVD+R 8X DVD+RW
http://www.newegg.com/Product/Produc...82E16827106287
$22.99
Your two already chosen parts plus...
AMD Phenom II X3 710
http://www.newegg.com/Product/Produc...8&Tpk=X3%20710
$125
GIGABYTE GV-R455D3-512I Radeon HD 4550 512MB 64-bit GDDR3
http://www.newegg.com/Product/Produc...82E16814125250
$44.99
BIOSTAR TA790GXB A2+ AM2+/AM2 AMD 790GX
http://www.newegg.com/Product/Produc...82E16813138143
$109.99
Western Digital Caviar Black WD5001AALS 500GB 7200 RPM 32MB Cache
http://www.newegg.com/Product/Produc...82E16822136320
$74.99
Microsoft Windows Vista Home Premium SP1 64-bit for System Builders
http://www.newegg.com/Product/Produc...82E16832116488
$99
OCZ Reaper HPC Edition 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800
http://www.newegg.com/Product/Produc...82E16820227267
$28.99
ENERMAX Tomahawk ETK405AST 405W ATX12V V2.2 Power Supply
http://www.newegg.com/Product/Produc...82E16817194037
$44.99
LITE-ON Black 22X DVD+R 8X DVD+RW
http://www.newegg.com/Product/Produc...82E16827106287
$22.99