Hello - I just had my first ultrasound - I'm 5 weeks 4 days but they measured me at 5 weeks and 1 day - I was really worried, but the doctor did not seem concerned at all......
I think it's a very good sign for you that there was already a heartbeat - I was told to expect a heartbeat only in another two weeks!
Good luck to you - this time is really stressful as we just want our babies to be ok and grow well, but try not to worry - it's really early and it's great you picked up a heartbeat! Also from what I understand the heart rate starts off very slow -so i think that's normal.
There's another medhelp forum
http://www.medhelp.org/forums/July-2011-Babies/show/974 where just pregnant women are discussing their first ultrasounds :)