in terms of using an Secure Digital High Capacity Card for a hard drive, Possible yes, recommended no.
Yes! It is possible to boot from an SD card, change your bios settings to boot from usb/flash/external.
SDHC Class 10 runs at 45mb/s (currently the fastest SDHC card)
Sata 2 (3.0gbit/s) 5400 RPM drives run at a maximum of roughly 1-90mb/s
USB 3.0 (3.2-5gbit/s) drives run at approx 75-320mb/s
Sata 3 (6.0gbit/s) 7200 RPM SSD's run at roughly 400-600mb/s (depending on quality)
Using the above data your best bet for a "cheap mans SSD" would be a USB 3.0 flash drive of sorts, but in all reality you won't really notice much difference unless you spend a hell of a lot on a external SSD defeating the purpose of the SSD by limiting it to usb but there really isn't much point in that unless your planning on taking your windows installation with you instead of in your pc.
To set windows installations to default on another drive is quite simple,
windows key + R, type in regedit push enter.
navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion
Change the Path in ProgramFilesDir and ProgramFilesDir (x86)
this will make it so by default your programs will install to another location of your choosing. x86 will only show if you are using 64bit windows.
Oh and if your thinking about raiding multiple 4gb flash/sd cards together yes that's strangely possible too, although the outcome is still no where near the results of an SSD
the below link shows 6x 4gb flash drives USB 2.0 getting a maximum of 75mb/s averaging 45mb/s still the same speed of the class 10 SD cards. and not even a 10th of what you get on an entry level SSD.
http://analogbit.com/node/4
So to answer your question, yes you can do everything your speaking of, although you would need roughly 109 usb 3.0 devices in raid 0 to even come close to the speed of an SSD (judging at 550mb/s) and even then the chance of one of them corrupting and requiring a full rebuild is extremely high, plus this would be going a tad insane just to set up, not to mention would cost more then the most expensive SSD to set up haha.
i would do it for fun and see how it goes(SD card not 109 usb drives) but i wouldn't recommend using it as a main OS install.
SSD prices have shot down quite a lot over the past few months, for example you can get an SSD for $59 AU OCZ Agility 3 60gb Max read up to 525MB/s, max write up to 475MB/s will be a hell of a lot faster then a 5400 drive and even a 7200 drive for that matter, i currently have one of these in my personal gaming computer, i paid $120 at the time last year in april. link below is of the ocz agility 3 60gb
http://www.pccasegear.com/index.php?main_page=product_info&cPath=210_902_909&products_id=17344
You will not regret spending a little bit extra to reap the rewards of an extremely fast storage device.