[low number] + [low number] = [slightly higher, though still relatively low, number]
Ya, that's just it. Some servers were hitting 200 people at certain times, so even with a merge you're looking at 400-600 people on. Some of the higher end population servers though should be able to hit the 1,000 player mark consistently over weekends and jp primetime.
Like Final Fantasy XI, the game specs will be extremely high for the time, but in about 5 years, an average machine can run it on max settings with little to no issues. Tanaka also expressed interest in making a benchmark program available.