Typically, people are using a single 120-128GB for their OS, programs, and maybe a few games depending on how much space is left. A more luxurious option (for me, at least) is a 256GB, where I have Win 7, lots of power-user type programs (Adobe Master Collection, Vegas, CAD, etc) and several games -- not every game really gets much benefit from SSD speeds, though. I keep Skyrim and some other games with lots of load screens and big maps. 256GB drives are getting more popular as prices drop. Personally, I've gone from 64GB, to 128GB, to 256GB, and I've been comfortable at 256GB for the longest amount of time. Other people will have different opinions, just depends on what you use your PC for. Mine's an everything PC, from web browsing and HTPC functions to video editing/encoding, some 3d modeling, and gaming.
I could easily see myself owning a 500GB+ drive to store more of my large project files for Premiere/Vegas and Autodesk 3ds on the SSD.
As for performance, the latest SSDs largely "feel" the same in real-world use, which will primarily take advantage of Random Read/Write performance. The Tom's Best SSDs for the money each month would be a good starting point in selecting one.