gaming rig under rs.25000 ( $450) excluding monitor.

First of all you have to be certain about some things before building your own PC. These are certain things you will need to build a gaming rig.
       1. Processor
       2. Mother board
       3. Ram
       4. Hard disk
       5. Power supply
       6. dvdwriter
       7. Graphic card
       8. Operating system

So here is the configuration for INTEL.

       1. Intel  3.3GHz LGA 1155 core i3 2120 processor @7.1k
     2. Asus mb p8h61 -m-lx @3k
     3. Corsair value select 4gb ddr3 @1.1k
     4. Seagate sata 500 gb hdd @3.8k
     5. Corsair CX430V2 @ 2.2k
     6. Samsung dvd writer 20x @0.9k
     7. Gigabyte HD 6770 1 GB GDDR5 @ 6.6K
     8. Windows 7x64 ultimate

All this mounts upto rs.24700. If you still want to reduce money then you can choose HD 6670 1 GB GDDR5 @ 5.2k and go with some local smps which comes with local cabin.

So, this is a decent gaming rig under 25k ( $450 ) which can easily play all new games at 1366x768 resolution.

Here is the configuration for AMD.

       1. AMD 3.6GHz AM3+ FX4100 processor @6k
     2. Gigabyte GA-880GM-D2H @4.3k
     3. Corsair value select 4gb ddr3 @1.1k
     4. Seagate sata 500 gb hdd @3.8k
     5. Corsair CX430V2 @ 2.2k
     6. Samsung dvd writer 20x @0.9k
     7. Gigabyte HD 6770 1 GB GDDR5 @ 6.6K
     8. Windows 7x64 ultimate

so this amounts upto rs.24900 and if you want to reduce the price follow the same  procedure which was stated above for intel configuration.
ENJOY.