Helping people with computers... one answer at a time.
USB Flash drives are useful for many things, but not for running your system from them long term. We'll look at why, and a Windows Vista alternative.
I see large USB flash drives are very cheap. My 'C' drive is partitioned to be 8GB, on which I have my Windows OS (Win 2K, in my case), and all my program installations. All my data files go on other partitions/drives.
So I'm thinking that for a little extra money, I could have my C: drive run off of flash Memory, instead of a hard drive (if the PC maker makes it possible). How much faster would Windows and all my boot-up programs load if this were the case (My current configuration take about 2-3 minutes for full reboot)? How much faster would Word, or Excel start up (not counting the file that it would be loading off of the accompanying HD, which would only be used to store data/document files)?
I often get questions where people want to use flash memory in various forms to speed up the normal operation of their computer. There's a lot of misunderstanding of flash memory and what it's capable of.
The good news is that while you still can't use USB flash drives for RAM, and you still shouldn't use them for your "C:" drive, Windows Vista has a new feature that will let you use them for something sort of in-between.
Why USB flash drives don't work as system RAM.
I've covered this before in an article Can I use a USB RAM stick to increase system memory?.
To summarize: system RAM is connected directly to your CPU on the motherboard via an extremely high-speed interface.
When the system accesses a USB flash drive it has to actually go through the USB device drivers and the comparatively much slower USB interface. Even USB 2.0 speed pales in comparison to the speed of on-board RAM.
In short, PC hardware simply isn't set up to treat an external device as true system RAM. And as we'll see in a moment, you don't really want it to anyway.
Why USB Flash Drives make bad system drives.
Flash memory wears out. The more frequently you write to a flash drive, the sooner it will fail. Eventually it won't be able to "hold" the data written to it.
You don't hear about it much, because the numbers of writes we're talking about are in the 10,000 to 100,000 range or higher. If all you're doing is periodically copying files to and from your USB flash drive, you can probably do that for years before you have a problem.
The problem becomes more apparent when you run applications that write to the drive a lot.
Windows definitely writes to the drive a lot. Windows is constantly updating the registry, swap files, and depending on how you use your machine, many, many other things.
If you were to use a USB flash drive as your system drive it might work for some time depending on how you use it. Booting from a flash drive often results in exactly this scenario. But using it constantly or planning to use it long-term is a trip down a one-way path: eventually the flash memory will fail.
This is another reason why you don't want flash memory as RAM: the system writes to RAM at a much more rapid pace than even the hard drive. Flash memory just won't last.
My earlier article, Can a USB thumbdrive "wear out"?, has more discussion of this situation as well.
Enter Windows Vista and "ReadyBoost"
So after publishing that USB RAM stick article last year, I started to get push-back. The common refrain was "You're Wrong! Windows Vista lets you use USB memory as RAM!".
No. Not quite.
There's nothing about Windows Vista that made the issues above go away.
Windows Vista does, however, include a nifty new feature that allows you to use USB Flash Drives to enhance performance in the form of something they call "ReadyBoost".
ReadyBoost is not RAM, and it's not really your system drive. It's something in-between.
What Windows does with ReadyBoost (and the feature it relies on, "SuperFetch") is in a sense, notice which programs you run a lot and pre-load them. To quote the Microsoft web site:
SuperFetch monitors which applications you use the most and preloads these into your system memory so they'll be ready when you need them.
ReadyBoost is nothing more than allowing SuperFetch to preload those applications into your USB Flash Drive.
The pre-load happens relatively infrequently; preserving the life-cycle of the flash memory. But reading the preloaded applications from the flash memory is typically faster than reading from your hard drive. Depending on how you use your computer it may improve performance.
But it's not RAM. And it's not Windows "running on" the USB Flash Drive.
Back to your question.
To actually answer your question: it's hard to say how much faster your system would be, that depends on too many things we don't know. However it's safe to say that it would be faster - until it crashed and died. Depending on the specific hardware involved and how you use your system that could be a day, a month, or if you're lucky, maybe a year.
But ultimately it's a game of Russian Roulette.
You'd be better served, in my opinion, by adding actual RAM to your system. It's possible that adding a second hard drive to your system and moving the system swap file to that drive, moving installed programs to that drive, or moving your data to that new drive might also speed things up for you.