That shouldn't have cleared all of your doubts. About the power supply he was saying if is the brand/model of your power supply that matters more than that it claims to have 400w.
Tell us the brand/model. If it is a good one it can handle the HD5770. If not you will either have to replace it or get the HD5670(or perhaps the HD6670) instead.
I have bought my pc in 2007-08, and i don't know what to tell about PSU.
But, the description on it says
Intex Smart Switching Power Supply
Model No. = IT - 2040
Pentium 4 ATX2.03 400 Watt
AC Input: 220V 4A 50Hz
So The OP Is From India, And I Am From India! The Intex 400W Costs 500Rs = 10$!
So Hope You Guys Get The IDEA Now!
@Sanjeevgopal: Look,Intex,Zebronics,Priya,Odyssey Etc PSU's are floating around in the indian market now. THey are cheap, real cheap, but they dont give out the amount of wattage they are supposed to!
Please Get A Corsair/Antec PSU , But They Will Cost You Above 2000 Rs (40$) Here!
I Got My Corsair 500W-V2(80 Plus Certified) For 3500 (70$)! Its Worth It Really!
That Is A 400W PSU, And Yet It Delivers Only 20A On 1 12V Rail!
12 x 20 = 240W! Thats Basically A 240W PSU Because its the 12V Rail Which Gives Off Most Of The Current!
Thats The Quality Of A Cheap PSU!