PSU... a Corsair, Antec or SeaSonic 550 watt PSU would be the best (max needed).
As for a gaming system... NO... The 9800GT isn't a good card to build off, specially at it's price. You would be better off getting an ATI 4850 and if budget permits get an ATI 5770.
ok what about 500W PSU????is that enough or am i supposed to go for 550W or more???and about the graphics card,i would like to go for nvidia products,so could u plss tell me the cards in their line-up better than 9800GT(keeping in mind the price)