If you want to do Cambodia, and if you want to do it first, here is what you can do. I just did an overland from Bangkok to Cambodia.
There is a bus from Bangkok's Suvarnabhumi Airport at 8:00 a.m. that will get you to Aranyaprathet or to the border with Cambodia, I am not sure which before 1:00 P.M. We did not know this when we did our trip, so we ended up taking a taxi to the Morchit (Northern Bus) Terminal, from where we took a 10:00 a.m. bus and were in Aranyaprathet by about 3:00 p.m.

An 80 Baht Tuk-Tuk ride took us to the border. We had already obtained e-visa to Cambodia before leaving the U.S. After exiting Thailand and entering Cambodia, we took a taxi for $45 that we shared with a young Korean couple to Siem Reap.We were in Siem reap by about 8:30 P.M. If you can catch the 8:00 a.m. bus from the airport, you can make it to Siem Reap by 6:00 P.M.

To catch the bus from the airport, you do the following.

1. AFter you leave the custom's area, exit the airport through gate 5 at the same level and catch the free Airport Shuttle to the Transportation Centre. There are two buses to the border. The first one leaves around 8:00 a.m. The second one leaves too late in the day to make it to Siem treap the same day. The fare, according to the sign at the transportation centre was 167 Baht. The sign might be old, as the fare from the Northern Terminal was 207 Baht.

Once you are done with Cambodia, make it back to the border in a raxi, for about $25 to 30. Take a tuk tuk to the bus station in Aranyaprathet from the Thai side of the border. The route from Khorat (Nakhon Ratchasima to Chiang Mai and Nongkhai is common upto Udon Thani, from where the two routes diverge. So your strategy should be to make it to Khorat, about 4 hours from Aranyaprathet, overnight in Khorat or take a night bus, if available to Chaing Mai. I have not done this part of the trip myself. You will have to do a bit of research yourself, but I think I am close to correct.
Chiang Mai is in the North and Phuket is in the Southwest. You might want to fly from Chiang Mai to Phuket to save the time and do the Phuket to Bangkok leg by land. Doing it all by land might take too much time travelling and make you rush through the sites. Siem Reap needs 2 or 3 days. There is Siem Reap thread . Check it out too. I paid next to nothing a night for a double room for an acceptable hotel. You might want to splurge and go with better hotels. The hotel I stayed in had A/C., hot water, attached bath and was fairly clean. Their going rate was $15 a night, but you can try to negotiate as I did.

If you need any more information