@s55
sir, i checked the links that you gave me. 620AS series is available in 500GB as well, but that is the NCQ series (Native Command Queuing) which provides better performance when there are multiple simultaneous read/write requests (good for server type applications but doesn't increase performance for gaming or regular use).
The 320AS series is also available in 500GB without NCQ, but the WD AAKS is a better hard-drive than that. the only reason that seagate is more expensive is cause marsons have priced it higher (kyon keh uun ka naam chal gaya hai).
Warranty for both is 1+2 years (ie 1 year comprehensive, and then after one year, you will have to pay a certain amount for repairs)