With the better SSD's offering at least 10 times the real-world performance of the best HDD's, the lure of super-fast Windows startup and application load times is hard to resist. Tom's Hardware has some interesting benchmarks on SSD performance and on HDD performance. Putting the OS on a 160GB Intel X25-M seems like the way forward here.
The Fusion-io ioXtreme is another interesting player here. Having one just for the pagefile and windows temp directory seems like the best use. While the review on Tom's Hardware shows some blistering performance figures, it also shows some so-so figures for the consumer version. It seems unlikely to have a big enough effect on frame rates to justify the cost. Perhaps the price of the professional version will come down enough for this to be revisited at a later phase?