the most cost efficient setup will be :
1) any motherboard supporting crossfire.
2) 2 ATI 5870 with eyefinity 6.
3) atleast 4GB of ram.
4) any good quad processor.
with this setup you can add 12 monitors(max).
if you are building this for gaming ,you need to have a i7 960 or a phenom IIx6 1090T BE processor......