1GB per player is only if each player has a large base. The memory load for 1 player - and I'm just talking the player/entity here, not their stuff nor the chunks they have loaded because of their view distance - is trivial. It's not the player that uses a lot of RAM, but the stuff they have built and the chunks they are loading simply with their view distance.
In the best case, you have something like an in-server convention where the players simply login, don't build anything, and stay in the same area.
In the worst case, you have every player spread far apart with a huge base in the Overworld, Nether, The End, MystCraft ages, etc.,. with a lot of chunks loaded and players flying around everything in full-speed MPS or GraviSuite suits.
Most servers are somewhere in between and of prudent planning would plan for the worst case. However, take 1GB per player as a "rule of thumb" and not an absolute.
Personally, 2GB is more than enough for me and my friend and we both build medium-sized bases in the Overworld, Nether, and MystCraft ages. Medium means 10x 36HP boilers, 20 Alvearies, an AE system drawing 300 units/t, and 6-ish chunk loaders each with an average radius of 6 chunks per loader.
Also note that 1.6 made huge improvements in memory leaks and - it seems like - memory usage. Your server memory requirements should actually go *down* in 1.6 with a comparable list of mods when compared to 1.5.2 .
