The Power Supply Requirement for your system is:
450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX technology in dual mode).
As other people has said before, the CORSAIR CX400 is a very good PSU which can handle your system. Anyway, for a few more bucks you can get a CORSAIR VX450 or an ANTEC EarthWatts 500W, which are better choices.
Anyhow, if you decide to save some bucks and go for the CX, you won't be disappointed.
Nice one guys , thanks for your replies. At the moment coz of budget limitation I will stick to corsair 400w but in future I do plan to overclock and use crossfire with 2 pci graphics cards, for that I will upgrade to a min 800+ W Psu.