Hey - Ive been travelling thru South East Asia for best part of 4months now So Ill give you some advise that might help you make a decision.
I started off in North Thailand (Chiang Mai) myself but your just planning on doing the trip the opposite way to myself so Ill reverse my trip for ya.
From Bangkok you can get a bus no problem to Siam Reap - this will first take you to the Cambodia border (where u pay around $30 Visa), and then the bus continues to Siam Reap. When in Siam Reap do Angkor Wat for 1-3days - the cost depends on how many days you want which also depends on how much cash you wanna handover - was around $60 for 3days that I did but that also included all travel, guide an sunset trip.
Then get another bus from Siam Reap to Phnom Penn - each of these bus journeys are an experiance cause the roads are mostly dust roads and 8hr longs but the buses are clean, aircon which make the journey easy.
Again get a bus from Phnom Penn to Saigon - yet again you will reach a border where you pay the average $30 for a Visa - I cant tell you the length of the bus journey cause I did a detour down the east coast aslong the Mekong Delta river.
When in Saigon - from your hotel or tourist office ask for the "Open Tour Bus" from Saigon to Hanoi - basically you pay about $25 which means you get a bus ticket from Saigon to Hanoi but can jump off anywhere aslong the way and they will drop you off at recommended hotels or guesthouses ( might sound dodgy but we stayed in 4star hotels for $5 a night - i was well chuffed - wasnt hostel backpacking but for $5 i didnt care) - places of interest aslong east coast of Vietnam - Dalat, Mui Ne (sand surfing), Nah Trang, Hoi An (my favourite place for beach parties), Hue - each of these bus journeys are anywhere from 6-15hours so be prepared..!!
Now I flew from Hanoi to Vientianne (Laos capital) cause the bus would have taken over 24hrs an after 4weeks of bus journeys in Vietnam I took the easy option for about $100
Laos is like Bob marleys house - everyone is soo laid back they are practical asleep its soo funny - lovely people an plenty activities - Vang Vieng ( river tubing, happy shakes) Laung Prabang (late bar is bowling alley till 2am) an Vientianne ( nightclub on 5th floor of aprtment block) - all transport is via bus an very cheap
Just remember that both Laos an Vietnam are still communist countries so nealry everywhere has curfews - mostly that you are back in your bed by midnight - now we used to just ask if we could stay out later to most of the hostel/hotel owners an they obliged but always ask cause most places pull the shutters down an you spend the night outside...not a good idea anywhere in the world
Hope this helps or it has confuzzles you even more oops
Anymore help gimme a shout
Declan (currently in Indonesia on way back to his 2nd full moon party in 3months)
PS - I still have the names of cheap places I stayed if you need