Yes, get a cheap SSD now, and put WoW on it. When you get a higher-end SSD, put the OS on it. I'd bet you won't see any noticeable increase in performance from WoW on a low-end SSD to a high-end SSD since the high-end SSDs generally only increase write speeds and large sequential reads - two aspects of the SSD that don't matter for us while playing WoW. The higher-end SSD is better for the OS since there will is always reads & writes going on with the OS swap space. Leave WoW and other read-heavy games on the low-end SSD.