We are planning to tour from Saigon to Bangkok and would love any advice on what to see / not see when we are there?

Would also love to hear from anyone who has completed this by bicycle

Haven't done any long distance cycling in SEA but know a few people who have done the 16 day Saigon-Siem Reap tour with Grasshopper. They had an amazing adventure and highly recommended it. If you're going it alone, you could at least check out their route....grasshopperadventures.com

Spice Roads, which does bicycle trips in VN is excellent. Take a look at them.

We did a private five day tour of the Mekong (from Saigon almost to the Cambodian border in May of 2008. It was a lot of fun as the company provided bikes, van, driver and bike leader. We could bike or ride in the van any sections. We'd stay in small hotels, usually the only one in the towns.
Our only complaint was with the bikes. They were all western "men's " style and too big for a couple of my friends. Next time, I'd allow extra time in the beginning to address problems with the equipment.

Thanks for all these.
Next thought - we were planning to take US $ - will that be advisable in all countries or do we need to take country specific local currencies?

Take US currency to exchange and an ATM card. Other than in Siem Reap, Cambodia you will use the local currency. Dong in VN, Riel in Cambodia, Baht in Thailand.

Oops - I hit reply too soon. You can't get Dong in the US, iy is legal tenser only in VN. Same with Riel. While you can get Baht in the US, you are always better off getting the currency after you arrive in the country.