You won't be paying PMI, since you are putting down more than 20%. If you put down 10%, then you would be paying PMI (which would be another $150+/month).
Calculating your payment should be fairly straight forward.
Go to this site:
Mortgage Calculator - Bankrate.com
$300,000 - $60,000 (downpayment) = $240,000 Mortgate Ammount.
$240,000 at 5% for 30 years is $1288 for your principal and interest.
Now you need to figure out your taxes and insurance. I am able to find the taxes on my local Assessor's web site, so I would try that route first. Around here the taxes are disclosed in the MLS listing as well. As for insurance, I would just guess $700/year. It depends where you are, so call your insurance agent or go to
www.geico.com to get a free quote online. Take your total yearly tax and insurance, then divide it by 12 which is your monthly rate. Add that on to the $1288 and there you go. Of course this doesn't include HOA if applicable and your closing costs. But in this market, you should be able to get your closing costs paid for by the seller (worth over $6,000 on a $300k house). So you should be around the $1500-1600/month mark depending on your taxes and insurance.