you guys can look into the APUs from AMD
CPU|
AMD A8-3870k(AMD Radeon™ HD 6550D)
|Rs.7400
Motherboard|
ASUS F1A55-MLX PLUX
|Rs.3900
RAM|??(you havenot mentioned)|??
PSU|
CX 430V2
|Rs.2563
Cabinet|get any generic one from frontech or iBall|Rs.800
Total||Rs.14663
Later if you want to go for a lowend discreet graphix card you can make it work with the 6550D in dual graphics mode(they used to call it hybrid crossfire in 7xx & 8xx chipsets ).
Only these GPUs will work with this processor in Dual Graphics mode HD6450, HD6570, HD6670.
This is a quad core processor slightly better than Athlon II x4 in performance.These A -series processors are very energy efficient.
Any gpu higher than that and the onboard gpu will shutdown.
Anything higher than those gpus and i will suggest to go with CORSAIR CX 600V2
else
you can go with Gaurav Bhattacharjee's config & add 6670 atleast
@op electricity bills can be a problem when you are running 20pcs & never buy 550Ti that's a inefficient power consuming card that doesnot perform well, better go for 6770
one more advice as you will purchase in bulk get advantage of that and negotiate a little with the dealer & you can get away with reduced price/component.