50/60GB minimum for a x64 bit build. If you don't have x64 then you don't have 4GB and memory is probably first to be upgraded rather than SSD.
100/120GB allows you a lot of space for games/apps. But not an infinate amount.
200/240GB allows you plenty of space, you shouldn't need to swap games out very often, but you will from time to time.
500GB means you've spent way too much money.
I started off with a single 60GB Vertex. Added a second and made them an array.
I then got a 100GB Vertex 2. So now I have the Vertex 2 as my main OS drive, and non-steam games go on that. The other 2 60GB drives have Steam games on it, but it does get full, so I can't have everything on it. I just archive the games off though to my storage array (RAID5 8x1TB WD RE2, on Adaptec 3805, on a server up my loft with WHS Vail and drive extender). Which means I can install them back faster than redownloading.
If you are buying a PC then I'd suggest just the £100 Vertex 2. Unless you have lots of money. The C300 is a dog at that capacity, it's only at the 128GB capacity and larger that it's good.
I'm just not sure about those hybrid drives. I'm nothing like convinced about them, and it's very difficult to effectively benchmark them to see how good they really are.