Rules of the road: Cambodians drive on the right side of the road. Traffic in Siem Reap may look chaotic. So watch first how the "locals" make left turns, deal with roundabouts...
Roads are mostly flat and paved well suited for the single speed White Bikes equipped with fenders
Weather: the average high is above 30 degrees Celsius (86 F) year round. The hottest month is April with an average high of 34 Celsius (93 F) and the wettest month is September with 276 mm (10.9 in) of rain. For more details consult WeathersparkorWiki.
Equipment. Come with with a hydration backpack or insulated water bottles; your helmet if you feel safer that way; a headlamp (the type you wear on your head) and a blinking rear light to be seen; a compass to identify positively the East Vs. West gate.
It is best to start riding at sunrise when it is cooler. A good "test" ride is to go the Roulous, a set of 3 smaller temples. It is 26 Kms round trip on good roads.
If you plan to bike to Angkor Wat for sunrise/sunset, you will be biking in the dark one of the ways with heavy traffic. Having a headlamp and blinking light becomes critical.
Consult a good guidebook such as "Ancient Angkor" by Michael Freeman and Claude Jacques to plan your trip to the Temples. It explains what you will see and gives a diagram of each site. Just be aware that "Ancient Angkor" was last updated in 2002.
Carry a map. The free "Siem Reap Travel Kit" available at guesthouses is a good map to have.
Take frequent coconut breaks to replenish your electrolytes. A coconut typically costs $1. Bring your own straw if you are concerned.
Some of the outlying temples like Banteay Srei, Kbal Spean ("river of 1000 lingas") are best visited by tuk-tuks or cars unless you want to bike over 100 Kms.
Banteay Srei is still worth a visit because of the quality of its bas reliefs and sculptures. See this account of a trip to Banteay Srei which includes a stop at the Land Mine Museum.