If you only have one hard drive, one cdrom drive, and onboard sound and video, and you aren't going to overclock you're probably ok.
The more drives and expansion cards you have in your system, the more chances there are that you're not goning to have enough watts. If you have multiple drives and/or a CD burner, a good sound card, and hot video card, you're gonna run out of juice.
I had people tell me I couldn't run a Celeron 433 on 145 watts. Onboard sound, video, 15GB 5400 rpm HDD, and 40X CD rom.
Rock solid stable, very cool, and very quiet.
It's working!
Good Luck