I agree with 40smama...don't give up! If there was no cardiac activity at 6,3 and then two days later there was a heartbeat of 73, it might have just started beating. And maybe that's why it was a bit lower. Best of luck with your next sonogram! I hope it shows a nice, strong heartbeat!
At 6w6d, my re's nuse said they like to see a hb of 100+ but don't give up hope - there's a woman over on the pregnancy 35+ forum (she sometimes posts here too) named kellym. She's pregnant now but at her pregnancy before, the baby's heartbeat was low and they didn't give her much hope but everything turned out great & she had a healthy baby in May '08.
I hate the waiting game - unfortunately you're in the middle of it but think positive - the hb could very well pick up in five days time.