As for choices in the 120 / 128 GB size:
http://www.tomshardware.com/reviews/ssd-review-benchmark,3115-6.html
Tier 3 (21 % faster than Tier 5, 33% faster than Tier 6, 61 % faster than Tier 8, 95% faster than Tier 10)
Mushkin Chronos Deluxe 120 GB
OCZ Vertex 3 Max IOPS 120 GB
Patriot WildFire 120 GB
Samsung 830 SSD 128 GB
Tier 5
Intel SSD 510 120 GB
Crucial m4 128 GB
Tier 6
Adata S511 120 GB
Corsair Force GT 120 GB
Kingston HyperX SSD 120 GB
OCZ Vertex 3 120 GB
Samsung 470 SSD 256 GB
Tier 8
Corsair Force 3 120 GB
Intel SSD 320 300 GB
OCZ Agility 3 120 GB
OCZ Solid 3 120 GB
Patriot Pyro 120 GB
Samsung 470 SSD 128 GB
Tier 10
OCZ Agility 2 120 GB
OCZ Vertex 2 120 GB
As far as how much space Windows takes up.....the box says 20 GB. Taking into account temp files, user files, program installs that force "common files" to C:\ , default mail storage locations, etc, a user without detailed knowledge of Windows inner workings could easily fill a 60 / 64 GB SSD in 6 months time.
My approach is use 120 / 128 GB Tier 3 unit as a minimum, 240 / 256 GB Tier 1 unit if ya can afford it.
I also suggest partitioning the HD for example .....
C:\OS
D:\Page and Temp Files
E:\Games
F:\Programs
G:\Data
Installing the OS to the C drive on the HD and having the rest separate in one or more partitions....this way when the SSD gets added, all the other stuff can remain as it. After installing the SSD, just install programs and games over themselves (not always necessary) to populate the Windows installation on the SSD with the necessary registry entries. This will maintain all tweaks and customizations from the original install. It also keeps teh stuff you want to go fast on the fastest part of the drive and the stuff you don't care about on the inner part of the drive. The outside portions are twice as fast as the inner allowing you to benefit accordingly.