You do not always need 300W+ PS. It more depends on how much stuff you have in your system than just having Athlon.
If you have more than a couple of 5 1/2 in drives, more than a couple of hard drives, multiple fans, many PCI cards, and fast CPU, especially Athlon, you will definitely want to get a 300W or higher PS. On the other hand, if you do not have that much stuff, 250W PS should be fine.
For example, my second system got a FlexATX case with 150W PS. It got a Intel 810 board, Celeron II 633Mhz, 128MB, V3 2000 PCI, a CD-ROM, two hard drives, a modem, and a case fan, and it is running more than fine. In fact, I have never seen any smoke coming out from it.
I would buy Antec 300W from places like Onvia or Buy.com if you decide to get one for cheap although I have to say that Enermax looks pretty nice.