help required!!

salam
i want to upgrade my pc,so i need assistance i havent checked makt thoroughly but as much as i hv inquired in mkt pc available in my budget are from 2ghz to 3ghz.
like
and a person offered me a deal which is
Processor 3ghz
motherboard:Intel D 915(is this a good board)
and i think thy r branded pcs so also tell me does it matter cuz they r i think much cheaper then self made ones
Ram:512
Rs:12000
and like that he told me that this board has a pci slot which supports the latest graphic card from 1g to onwards .so is pci necessay (or wht is pci?)
so can u ppl plz help me to decide which board i shoud buy and how much should be a processor.As i want to upgrade my system to play game so consider it while giving advice.
is 1g graphic card enough to play the new games and which card is best(brand).and how much ram would be appropriate.
can i buy a pc whith little less speed processor like:2.4ghz or 2.6 , 2.8ghz and install a 1g graphic card,1g ram would the pc be able to play heavy games or high speed processor effects the performance of games much.

Please post in the kampootar and IT forum