How to build a reasonable computer?

I am looking at putting computer parts together for myself. Main purpose of the machine is to help out with general work and office tasks. No cutting-edge machine but it should be made with modern parts, at least extensible and upgradeable. I am not a gamer but I might be hooking the system to the TV, as a media centre.
I can afford to spend between $800 and $1,150 but not more. I need help working out which parts are compartible with each other and selecting those that will best suit my need. Please choose computer parts from the list below.


MOTHERBOARDS
Gigabyte GA-A75-D2H---$98
Gigabyte H67MA-USB3-B3---$139
MSI P55A Fuzion---$169

PROCESSORS
Intel i3 540---$109
Intel Xeon 3040---$140
Intel i5 2320---$193

Intel i7 2600---$339
AMD A8-3850---$149
AMD Phenom II X6 1100T---$206

CASE
Cooler Master Elite 100 mini-ITX [150W]---$79
Cooler Master Centurion 590 [500W]---$108
Antec NSK2480 [380W]---$123
ThermalTake Armor+MX---$140

HARD DISK
3.5" HDD
Western Digital Caviar Blue 160GB IDE---$79
Western Digital Caviar Green 3TB---$180
Seagate Barracuda XT 3TB---265
Seagate Constellation 2TB SATA 6Gb/s275

2.5" HDD
Seagate Momentus XT 7200 320GB---$105
Kingston HyperX SSD SH100S3/120G---$265

MEMORY
DDR RAM
Kingston 1GB 3200 [KVR400X64C3A-1G]---$48
Corsair 4GB 800 Kit [VS4GBKIT800D2]---$65

DDR3 RAM
Kingston 8GB 1600HX [KHX1600C9D3K2/8GX]---$75
Kingston 12GB 1600HX [KHX1600C9D3K3/12GX]---$109

SERVER RAM
Kingston 4GB Kit 1333 ECC Unbuffered [KVR1333D3E9S/4GI]---$99

GRAPHICS CARD
AGP Graphics
Gigabyte 1GB 4650 AGP [GV-R465D2-1GI]---$115

Nvidia Graphics (PCI-Express)
Zotac 512MB ION [x1]---$69
MSI 1GB GTX560OC Ti [x16]---$269
Asus 2GB HD6950 810MHz [x16]---$329

OPTICAL DRIVE
Liteon IHAS324 DVD-RW--- $24
Liteon IHBS212 12X Blue-Ray LghtScribe---$105