Do you have RAM and HDD..???
If yes, then which RAM..???
The cheapest combo possible IMO is
* AMD AthlonII X2 250 -- 2.6k
* Asus M4N68T-M -- 2.4k
Total -- 5k
The mobo is DDR3 supported.
If u have DDR2 RAM then u have to look for another mobo supporting DDR2.
You can also have a look at the intel Atom combos at 3k-4k. They are also less power consuming and are very good for internet usage.
If u r running ur PC 24*7 then u should also have a good PSU like FSP SAGAII 350W at 1.5k.