Buying an SSD will be counterproductive considering the latest blockbuster PS3 games are on average 20-30GB for one game. It gets even worse. The recently added Uncharted 3 to PSN takes up a whopping 87GB! The download size most of the time is compressed and it takes up nearly double the space once it's installed.
PS4 games are rumored to be around 50GB for one game and that's not including any potential DLC addons. One game could take up 100GB if it's a huge game with multiplayer and lots of cutscenes. I imagine that Sony's plan is to eventually stream digitial PS4 games over the cloud so you won't have to download the entire game. Maybe just a little bit of data and the rest is in the cloud. I can't imagine them expecting 500GB to be sufficient as the only capacity for a next gen multimedia device otherwise.
Switching to SSD just limits your storage space options tremendously and will cause you to run out of space faster despite spending a lot more money per GB. As mentioned before the better option is a 1TB Hybrid drive like the Seagate Momentus XT that I use on my PS3. It performs nearly as well as a true SSD without sacrificing storage capacity.
I have a Momentus XT in my PS3 and the menus and games run silky smooth. Everything loads a lot faster and it feels like I'm using a genuine SSD drive. Plus unless PS4 has the proper hardware and software to take advantage of SSDs it probably won't perform any better than Hybrids.
Seagate 1TB Hybrid Drive $129.99 - http://www.bestbuy.com/site/1TB+Internal+Serial+ATA+III/Serial+ATA+II+Solid+State+Hybrid+Drive+for+Laptops/8808633.p?id=1218897640424&skuId=8808633&st=Seagate%201TB%20hybrid%20drive%20laptop&cp=1&lp=1
