Even the lowest capacity SSDs available are way to expensive for my taste. Good luck buying a decent one cheaper then what you paid for your car.
A better solution would be to get a uber motherboard that can handle having 24+ GB of DDR3 ram shoved into it. A huge a** ramdrive and your good to go.
My current mobo can handle 12GB of ram. But I know for sure there are mobos out there can can go 24 or maybe higher. I haven't checked in terms of what software would be needed for a ram drive that can use a good portion of that. And I don't know if said software would be good enough yet to allow your paging file to be dropped into a ramdrive. Not something to run your OS off since obviously a power outage or simply shutting down would result in any changes being lost.
Despite possible software limitations, ram drives in theory will always be faster as it uses your ram and that is the the next hardware component closest connection wise to your CPU, so it will always be faster then going through a hard-drive port or USB.
EDIT:
For those with money falling from every orifice of their body, they could go real hardcore buy this monster:
http://www.hyperossystems.co.uk/07042003/hardware.htm
This bad boy claims to use DDR2 ram as a hard-drive and supports up to 64GB of ram put into it and features battery backup in-case of power loss and external power source since it won't be powered if the computer is off. Plug it into a Uninterruptable Power Supply and you got double power loss protection.
Techinically this would in theory make it faster then any of the flash based SSD drives out there right now as nothing is faster then DDR2/DDR3 ram in terms of seek time and write/read rate ratio.
But unless you got $449 for the drive and the perhaps thousand plus dollars for the ram to max it out at, I wouldn't recommend it.
Though the 48GB model is cheaper by 200 bucks.
Oh and need I mention you can probably raid the f***ers? Now that would be uber overkill.
If you can find 48/64GB worth of DDR2 ram at a good price on the internet somewhere, it might actually be worth getting. Just don't buy the ram from the same site as that drive. I'm pretty sure they are overcharging on it anyway.