(1) Is it necessary to purchase additional graphics card other than the main Intel processor? If so then which card is good?
>> If you are not a gamer or graphic designer, builtin graphics is enough. Otherwize you need to go for some graphic cards. I do not have much info about latest cards available. Let us wait for somebody else's response.
(2) Which is the latest Intel processor?
>> If it is a home PC, Intel Core 2 Duo is there. Choose some vPro version if it is for business.
(3) How much RAM/hard disk capacity I should purchase for Vista/VS 2008.
>> I hope you will install Windows Vista. Then 2 or 3 GB RAM is good. Hard Disk 160 GB is enought. But if you have more budget go for 320.
(4) Is there any use in having 2 hard disk of 500 GB size?
>> If you plan to use two 500 GB HDDs, you can seperate OS and other applicaitons. But this is very big in size for a home PC. You must check with the vendor regarding the issues. I think you need to put a 750W SMPS for this. If you plan to use this as a backup device, then USB based HDD is good.