Quoted from Paul Thurrot's site.
5. Windows ReadyBoost
It sounded like science fiction the first time I heard about it: Windows Vista includes a feature called ReadyBoost that lets you use a USB 2.0-based flash memory device--typically a USB memory key--to speed up the performance of virtually any Vista PC. After grinning like an idiot for an uncomfortable amount of time, I realized the Microsoft rep telling me about this feature was serious. Huh. Hmm.
Now, there are prerequisites, of course. The USB memory device must meet certain performance and storage characteristics (2.5MB/sec throughput for 4K random reads and 1.75MB/sec throughput for 512K random writes; 64 MB to 8 GB of free space; 256 MB of overall storage or more) which virtually no human being understands or knows how to discover. Don't worry about that stuff too much unless you're literally shopping for a new memory key: Just plug one of your exiting keys into a Vista machine and see if it works.
ReadyBoost works by augmenting your system's memory with the memory on the USB device. The first time you plug in a compatible device, the Vista AutoPlay dialog will present "Speed up my system" as an option (Figure). You can then access the Memory tab of the device's property sheet to determine how much storage space to set aside for ReadyBoost (Figure). Note that this storage will no longer be available for other uses (unless you format the device or change the properties in Vista). You can't use a single key on two or more Vista machines. You can't use two or more USB keys on one PC. And, yes, Vista will recommend how much to set aside. (It typically wants a lot of space, so it may be a good idea to dedicate a USB memory key entire to this project. Hey, they're cheap.)
Why is this good? With a typical desktop PC, it's not necessarily hard to add RAM, but that's not true of most notebooks, and in either case, you may be limited by technical ability and desire, corporate restrictions preventing you from mucking around with your system, or actual hardware limitations where you've already maxed out the system RAM. In any case, you can simply improve the performance of your system by plugging in a compatible USB memory key, configuring it, and getting back to work. Bliss.
There have been some concerns that ReadyBoost will shorten the lifespan of the USB memory key because they're only rated to a certain number of reads and writes. My attitude is, who cares? These things are cheap, and if you're wise, you'll dedicate one solely to this purpose. If it dies, it dies. Get another one. (Besides, Microsoft refutes these claims, noting that its research shows that you will get at ten or more years out of life using ReadyBoost.)
ReadyBoost will give the biggest improvements to low-RAM PCs and, of course, the more storage on the key the matter. Adding a 1 GB USB memory key to a PC with 512 MB of RAM (ugh) will provide dramatic results. Adding the same key to a PC with 4 GB of RAM, well, not so much.
So the bottom line is this. ReadyBoost is a set-it-and-forget-it feature with no downsides at all, a small financial outlay (assuming you don't have a compatible key sitting around; I understand they come in cereal boxes now), and it delivers a nice performance boost. Who could ask for more?