Suppose i have a computer system worth $600 which i purchased on 1-jan-2012 and now i have purchased more RAM on 6-7-2012, how to do this kind of transactions properly. How to add value to an asset? Should i add it directly to Asset without mentioning addition or date?
The answer is "it depends". From accounting perspective you do not increase the asset value just because you bought some gadget or peripheral to your laptop. You only do that if you've increased the useful life of the asset. If that's so you add the value added, and recalculate the depreciation based on the new expected useful life.
The date is important, because that's when you reset the useful life "clock" - stop the previous depreciation schedule, recalculate value, and start the new depreciation schedule.
Talk to your accountant for more details and talk to your IT guy about whether the useful life of your computer is expected to increase significantly (= years).