hi folks.. i am gonna get a new arduino board. so one time investment, i have chosen arduino ATmega2560.
will that be good, or please suggest me what will be the best
There is no 'best', only what best meets your requirements and budget. We can't know your requirement and budget restrictions.
You are asking questions only you can answer correctly.
Lefty
well thanku sir 
i dunno exactly wat to buy. budget is not a problem coz we as a group gonna get it for project. When i went through the net, Arduino mega2650 was the maximum range available. so only selected that one.
As a 1-time investment, I think I would go after something more modular than, so that if a part gets damaged, it can be replaced and the rest reused.
The Mega2560 has no such modularity - all surface mount, the 2 main chips that can be damaged are not user replaceable.
I think a small collection of parts, such as an AVR ISP programmer, an FTDI Basic or a CD2102 Module, and 644/1284 based design like a Sanguino would be better. Get 2 USB/Serial modules even, the 324/644/1284 processors have 2 serial interfaces.
Sanguino in India: http://www.bhasha.co.cc/ (opens really slow in the US)
Example of the other modules:
FTDI USB/Serial:
http://www.ebay.com/itm/FT232RL-USB-UART-Breadkout-Board-RS232-FTDI-Convert-/130559536622?pt=LH_DefaultDomain_0&hash=item1e65f46dee
Connect Pwr, Gnd, Rxd, Tx, DTR to same pins on Sanguino to download sketches
CP2102 USB/Serial:
http://www.ebay.com/itm/1Mbps-USB-TTL-COM-Converter-Module-buildin-in-CP2102-/330606172980?pt=LH_DefaultDomain_0&hash=item4cf9a9db34
Cut the trace that goes to the Reset pin, wire the pin to the DTR pin.
Connect Pwr, Gnd, Rxd, Tx, DTR to same pins on Sanguino to download sketches
AVR ISP programmer:
http://www.ebay.com/itm/51-AVR-ISP-Programmer-Downloader-USBASP-ATMega-ATTiny-/170622588574?pt=LH_DefaultDomain_0&hash=item27b9e6169e
Connect Power, Gnd, Reset, MISO, MOSI, SCK to the ICSP pins for burning Bootloader onto replacement 324/644/1284 chips if you should happen to kill the chip supplied with the board.
Now you have the tools for basic development. And your next project can be built up from just a Sanguino board, or from your own design.
sir u have given so many boards. but i dunno wat these boards are. i know to program only in arduino since i had a workshop on it in our college. In that case which arduino board will be better, like what u have said is ryt. i cannot replace the components in Arduino mega 2560. so what are the other choices...
thank you
The Sanguino as the basic Arduino type board, so you have 2 hardware serial lines, and more I/O lines. That's the processor part, and the part most likely to be damaged when you testing, developing, etc.
An FTDI Basic for downloading sketches. That's the serial interface part that you would connect to your PC. Typically you would connect this, download your sketch, than disconnect & let your project run. (Then when 12V is accidentally applied in the wrong place, the processor is fried, and this part is okay 8) )
Or leave it connected while you monitor Serial outputs for debug/print statements.
The AVR ISP programmer, that is only needed when you want to put a bootloader on a new blank chip.
That particular FTDI breakout board can also be used to install a bootloader. You would connect the CTS, DSR, DCD, and RI pins (and power, ground) to the ISCP pins per this technique:
http://www.geocities.jp/arduino_diecimila/bootloader/index_en.html
Study X3 connector on the Duemilanove schematic and you will understand what I mean.