It is a great idea to put windows on a smaller SSD and then run a RAID for your working or scratch disks. RAID gives you the best bang for you buck right now in size vs performance vs price. A 4 drive RAID 0 with some used 160 GB HLFS drives on ebay will run you about $120 giving you about 600GB usable space and very fast throughput; probably in the 200-300 MB/s range. But a RAID 0 with 4 drives will probably run a single failure every 1-3 years that you need to plan on so just buy a fifth drive for a spare for another $30.
HDD's do well when you read and write larger files like video files. SSDs work good at that as well, but they really shine when accessing 1'000's of small files like windows has. And when there is that much of a price premium, you don't want the performance potential to go to waste like on larger files that can be efficiently served by spindle HDD's properly setup.
The biggest caveat of the RAID 0 is if ANY drive fails, you lose the array. So you need another drive as a backup drive that you sync daily or more often via a program like sync toy or free file sync so when you lose a drive (it will happen) you do not lose your work.
Even with a quality used RAID card and a 1TB drive to backup the raid, you still are upwards of 1/3 the price of a comparable SSD. And a 1TB+ usable array using 300GB HLFS drives would only add about $80 more.
RAID 5 is great for read speeds, but writing is slower because of the parity it has to update and/or write so it would not fit your needs well because you will have both alot of reading and writing. A RAID 10 would give you redundancy so you could backup or sync less often but the number of drives doubles for the same performance. And not many computer cases have room for 8 drives; also a typical 4 drive RAID 10 will give you half the throughput of a 4 drive RAID 0.
As a note, to keep throughput at its maximum, you will want to purchase drives as identical as possible, at least using the same model of drive. When performance isn't an issue, you can mix drives but performance and life of the HDD's will be slightly degraded. And from my research, used HLFS drives are about the fastest (throughput, not spindle speed alone) cheapest drives that can be had right now.
SSD in the corporate IT world are being adopted for their performance, but they still are too pricey to be viable for mass storage needs so spindle hard drives are still the the norm. So unless you have money to burn, get some fast used HLFS hard drives and a quality 3 ware, Areca, or similar 4-8 ports used raid card.
Then don't forget the Firepro or Quado graphics card and lots of RAM!