I work 9 hrs/wk on W2. I started 2nd job 2 wks ago (15 hrs/wk) on 1099. Do I have to pay taxes on 4/15/08?

The calculation is based on your expected total tax for the year divided by 4. There's really no "set amount" and you can adjust your payments as needed based on income. If you want to start making estimates in June remember it probably will be more than $500. If you expect your tax liability to be $2000 for the year and you are now dividing it over 3 payments intead of 4 you'll have to send in extra.