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.
Yeah, I have a Samsung 840 pro SSD that is 128 GB. I really would recommend getting it for your OS. Right now, I have around 72 GB left, but I store almost everything on my second hard drive, with shortcuts to your desktop.
SSDs are really useful for heavy weight editing. If you edit just a bit, I would go with regular HDD.
As Stephen said, some games like Skyrim benefit from a SSD, but it is not worth the price in my opinion to buy one over an HDD for gaming purposes.
I chose Samsung SSD over Vertex simply because they were neck and neck, and I liked samsung as a company
I would go for one bigger drive, because usually the SSD with the smallest capacity in its series also has the lowest performance. Often you get the best performance for 250 or 500 GB drive.
I would go for a Samsung 840 Pro if you search maximum performance or 840 Evo, which also provides nice performance but it is meant single user environment, and also got a fair low price.