Should i build a new system

Depends what your budgets going to be. Sandy Bridge is due out in january, bulldozer isnt due out for a while, but if you are building a lower budget system it doesnt really matter was both will be out of your price range. If you are considering getting a higher priced LGA 1156 system then you may want to wait until Sandy Bridge comes out, bulldozer wont be out for several months yet.
 

fpsblue

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Let say if i buy a AM3 system would it be obsolete when the BD comes out?
 
No longer top of the line, yes; obsolete and unable to do stuff, certainly not. It will be just as powerful when bulldozer comes out as it is before, it doesnt suddenly become obsolete because its not the shiniest toy on the block anymore. Its the same as thinking that an i7 975 became obsolete once the 980X came out, it would still destroy most tasks.