The sata protocol adresses a 10bit length for a byte, making 3Gbit 300Mbyte and 6Gbit 600mb/s maximum transfer speed. On any other part a byte is 8 bits
Install your most used things like your web browser, music player, etc on it. I have one game on my SSD right now, and it does make a difference over having it on a regular HDD.
What I was saying about Steam earlier is, it relies on the steamapps folder within the Steam folder. I’m sure you either would want more installed than your SSD would be able to handle, or you would want a huge SSD. Huge SSDs are pretty costly. That link is to the cheapest 512 GB that Newegg has. You can get a 1TB hybrid for about half the price, but that’s part flash memory, and part spinning disc.
I tried tricking steam by mounting a partition directly to C:\steam\steamapps\ by using diskmgmt.msc but it didn’t work
I could have sworn I read something about a program that lets you install a few games in one place while the others are in another place. I suppose it doesn’t matter since BF3 is probably the only game I’ll do this with and I forgot that it uses Origin 
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I got myself a pretty high end PC a few monthes ago, I took a Crossfire of 6950hd’s but still I wondered about the SSD.
After reading multiple articles & asking a few friends, I ended up buying a Vertex 3 of 120Go. My friends were all like :" not worth the money blabla"
Well its absolutly worth your money. My computer boots under 30 seconds with full windows loaded and ready to launch new programs, Even with the heavy-ass Kaspersky antivirus I have, game loading times are only a few seconds if none, you will always be first in any Multiplayer game to have loaded the games and everything generally is faster.
The only thing you have to consider is that 120Go is really not much, but going for more will rip your wallet. With windows and 2 or 3 big games your are pretty much full.
I do not have an SSD. But, they are very useful if you know how to use them. I would install the basic programs on them such as Windows, Web browser, media players, etc. But other programs you wouldn’t use everyday, such as games, use and HDD. And I mean the games themselves, Steam would probably be fine. Just not the games themselves. They really will help your speeds but be careful to not save too many things to your desktop or other common places. As you said, they’re pricey and have little space as well so don’t waste TOO much money on it. That money could be spent on a bigger HDD or other components in your PC. I’d say anywhere from around 30gb-50gb should be enough.
I was actually going to get another HDD but want to wait until the prices go down. I have $200 that’s burning a hole in my pocket, but I’m not sure if I want to get one for around that price, or like you say, a smaller one. Decisions decisions.