power supplys come in two styles, AT and ATX. Look on your old powersupply to see which type you have. It should have a name plate on it.
Cheap isn't always best. You can find a 600 watt for $50, but it just won't push a solid voltage all the time and could cause you trouble. Also, spending a bit more now could save you money down the road. If you plan on keeping your computer for awhile and upgrading it a bit at a time, getting a 600-650 watt now will be able to handle bigger video cards down the road. Spending $75 now for a powersuppy, then having to up grade to a 650 next year at $100 just wastes money.