Cost vs performance.
As All Have indicated, the SSD is About 40-> 80 Times faster than a HDD.
However; That said, and SSD will:
.. Considerably improve boot time - that is going from "start loading OS" -> able to open first program. Also shortens shut down time.
.. Speed up program load times - How noticable depends on program.
.. Will load any files much faster than a HDD, providing the file is on th SSD.
Will NOT:
.. Speed up web surfing, downloading files from the web, nor speed up email
.. Will Not make programs RUN faster. Does not improve FPS in games (Will speed up map loading providing the map is on the SSD.
.. Will NOT speed up file load times if file is on HDD
Added cost, since a HDD is still need, is about $100 for a 128 gig SSD, or $170 for a 256 gig (on sale). There are cheaper version but generally do not recommend.
On Size:
.. Currently the Min size that I recommend is the 120/128 gig SSD. I DO NOT recommend the 60/64 gig SSDs
.. Generally Windows 7 OS + programs take about 35 Gigs. Add to this any games you plan on installing.
.. For a 128 gig SSD. (1) Formated size is about 119 Gigs, Difference in way manuf count vs the way a computer counts. (2) Then you MUST leave an amount free so that Wear leveling, Garbage Collector (CG), and Trim can work here magic to keep the drive near manuf performance specs. This started out at 10%, then jumped to 15% and now the recommend space to leave free is 20->25%. This Now cuts the usable space down to approx 90 -> 100 gigs Available (depending on 15% vs 25 %)
Ref on "free space":
http://www.anandtech.com/show/6489/playing-with-op
I have In-use two laptops and three desktops, ALL Have SSDs + a HDD except one laptop - Would Never go back to just using a HDD.
Bottom Line, if You can afford the extra $100 go for SD + HDD. Remember the HDD is the single Biggest bottleneck in system performance.