2x2GB should not cost more than $100. I would start at 4GB, I have 4GB and I see spike usage of 80% runing starcraft 2 on Ultra settings. I would think ffxiv would use more than SC2 but I don't get to test it until 9/30.
That depends on the quality of the RAM. I see many people who say this kind of thing neglecting things like RAM Timings, CAS Latency, Speed, and Heatspreaders which are all important to the quality of your RAM.
For instance this RAM: http://www.newegg.com/Product/Product.aspx?Item=N82E16820146748&cm_re=mushkin_ddr3-_-20-146-748-_-Product
This is 1333 RAM, with a CAS Latency of 9, timings at 9-9-9-24, and no Heatspreader is cheap but in no way is it quality RAM.
This RAM from the same brand is still not the best, but much better: http://www.newegg.com/Product/Product.aspx?Item=N82E16820226123&cm_re=mushkin_ddr3-_-20-226-123-_-Product
1600 RAM, CAS Latency 6, timings at 6-8-6-24, and a good Heatspreader as well.
Keep in mind though that as ram speed increases (1333, 1600, 2000) and also as you go from DDR2 to DDR3 timings will generally increase, but that doesn't mean that DDR2 is better than DDR3. Also DDR3 works best in a triple channel configuration, while DDR2 works best in a dual channel configuration.