ReadyBoost drives are nice, if you have a spare USB header inside your computer and you get the right one. Many of them tout huge performance numbers but actually perform like crap.
Also, the comparable price for actual RAM now makes ReadyBoost drives less worthwhile. Picking up 4GB of real DDR2 for $20 on sale would yield a much better performance increase than $15 for 4GB in a ReadyBoost drive. Also, I believe it's still the case that the biggest addressable memory space for ReadyBoost is 4GB, even if you have a 32GB drive handy. The biggest benefit you'll see from those is OS load times, that's all they're really great at. To make Vista come back from power down quicker.
There's also hard drive hype in the form of solid state discs and the Western Digital Raptor line. I run a 150GB VelociRaptor as my boot drive, and I honestly see no benefit to it. It shaves a few seconds off my application load times and cost me over $1/GB. Solid state discs are even worse in terms of perf/$. Me, I'm stupid when it comes to my computers. I'll buy ridiculously overpriced stuff and even overclock the heck out of it just to squeeze every raytracing second out of Max, for example. But, for emphasis, I'M STUPID.
Now I can't only say negative things. For someone with 2GB or less RAM, no open RAM slots, running 32bit Vista, and an available USB2 port or header, it's a fantastic option to increase performance.