I am building an arducopter (quad) with telemetry and FPV system and these are the specifications can you guys suggest any modifications ? or is this build OK
frame- ST360
motor- 4*Avionic C2830 KV850
ESC- 30A Emax with simonK 
props -  10x4.5R
FPV   -  5.8G FPV Video TX RX 200mW+11db antenna +720 PAL Camera (SONY)
Flight control- APM2.6 + Ublox NEO 6M GPS Compass w/ Foldable Stand  + Power Module APM
Telemetry -  3DRobotics 3DR Radio Telemetry Kit 433Mhz Module
Battery - wolfpack 4200mah 20C
TX/RX-  Avionic RCB6i 2.4Ghz 6ch transmitter with receiver
                                                                                                        any more modifications ? change in parts required?

All looks fine check system Total weight . If you can afford more powerfull motor.  which can give u approx. 1.2KG per motor.
Motor Efficiency goes down with age so I suggest go for better motor.

upgrade the frame to a bigger one.

yup - you could go for a 450 frame - motors, I think are perfect - lower Kv  - high stability - high efficiancy.

You may upgrade to better motors and a bigger frame.

motors are good.

I meant, which gives higher thrust.

well I don't know what is the thrust of RCB motor, but a 850Kv 2830 should give more than enough thrust for a 450 quad, unless you tie a brick to the quad