i have a gts 250, these are essentially the same cards. i would go for at least 400 - 450 watts, presuming you use one HDD and 2 Disc drives.
I see you have a slimline PC: check the size of the card, and see if it will actually fit. Previously i have made the mistake of buying a too big a card.
further, make sure that you have pci-e power connectors, on your PSU. if not i think you willl be fine sparkle usually sends products with molex to pci-e power adapters. (at least i got them: Sparkle GeForce GTS 250 512MB)