What Power Supply do I need?

I'm wondering if there's a guide anywhere that will tell me based on componants I have, how much wattage I'm using, and what size power supply I should have. I know for example, an Antec 350 watt powersupply, and a no name brand 400 watt power supply, would probably give the same performance because the cheapo one wouldn't be as high quality as the Antec one. I don't have a TON of stuff... but I check on a web site that gave approximate wattages, and I added up everything I could, and it came out to about 260 watts, which doesn't seem right, since I have an AMD XP1700+, 512 mb DDR ram, GF4 Ti4200 video card, IDE DVD-ROM, IDE CDRW, 7200 RPM IDE hard drive, NIC, sound card, floppy drive, 4 case fans, and 2 USB devices that use about 15 watts combined.