I don't have SSD, but I have 60Gig used, for Win7 & CS5 MC & MS Office & other smaller programs on a regular drive
I did configure Win7 to put the swap/page file on a different drive, and followed advice here to set temp files to a different drive
Drive C space http://forums.adobe.com/thread/1007934?tstart=0
If you don't use your SSD for any temp files, 128 should be plenty
I have only 46 GB on my system drive - less than John, but I have only Premiere CS5.5, Audition CS3, and Photoshop CS5, rather than the full Master Collection.
So on the face of it, 128 GB should be fine. It is a lot cheaper as you say, but due to the way SSDs work, 256 GB will be faster. I bought 256 GB for that reason.
Repeated writing does not improve SSDs, so keep your page file elsewhere. I have the entire My Documents file and E-mail on another drive for this reason - so only system files are on the SSD.
Suggest the 128... It is large enough - unlike rotating drives you can have SSDs almost full and their performance doesn't massively degrade. And regarding speed, read speed is the same as the 256GB and write speed is just not that important for the boot drive when configured properly and Windows page file pointed to a fast rotating drive or array.
Also, if you want to maximize the performance and pay the price of a 256GB SSD, get 2 128GB's and RAID 0 them. Before the RAID 0 "boot drive" naysayers reply: 1) thou shalt make a good image backup of your boot OS/programs drive for a quick restore without having to rebuild your whole system (this is true for single drive, RAID whatever, and RAID 0 configurations alike IMHO) and 2) I've been using a RAID 0 OS/programs configuration for my desktop PCs for over 10 years now and have yet to have any data loss (choose good and compatible drives, set it up carefully, and enjoy the speed!).
Finally, don't forget that your OS/boot drive or array will have very little effect on how well or fast Adobe Premiere Pro performs after it has "loaded". I personally love the speed SSDs provide, but it is really for other PC uses, not Premiere Pro (the subject of this hardware forum).
I'd argue that 128 GB should be plenty for a System drive on an edit rig, be it an SSD or HDD.
dont forget to turn off all kinds of defragmentations from any anti viruses for example! as it may shorter the life of SSD