K800i not available in India.
All the 3 are same.
K790i and K800i have exactly same design. K790i has EDGE but lacks 3G whereas K800i has 3G and it lacks EDGE.
K810i has a different design. It also lacks EDGE but has 3G. K800i = K810i.
All the 3 have the same camera driver and internal software.
Its upto your taste or requirement of EDGE/3G, which one you want to have.
I'll say K790i because it has EDGE. When 3G will be launched in India, it is surely gonna be expensive and EDGE prices will become affordable. Thats why EDGE.