ridge and ridgeless refer to a ridge that runs around the circumference of the basket (have a look at your silvia double basket, it has a ridge about 1cm below the top of the basket) which allows the porterfilter spring (a "C" shaped bit of metal) to hold the basket in place. If you have a ridged basket you can use a weaker spring, if you have a ridgeless basket you might need to get a stronger spring to stop the basket falling out when you hit it to get the puck out. No big deal in terms of the mechanics of keeping the filterbasket in place, but people have preferences with how that affects the tamp and thus the coffee.
Silvia uses a 58mm basket so i figure 58mm baskets would fit (Im not totally sure). I think if you do a search you will find that the popular contenders for use in the Silvia are the
-LaMarzocca double ridged filterbasket
-LaMarzocca style double ridgeless filterbasket (not actually made by LaMarzocca)
-Synesso double ridgeless filterbasket
I might not be the best person to recommend baskets because im a Silvia newbie, but Ive progressed from the silvia double filterbasket to a LaMarzocca double ridged filterbasket and have just ordered a Synesso double ridgeless filterbasket. I found the Silvia double FB tricky to get good shots with and the LaMarzocco has annoyed me because it has a smaller diameter below the ridge, so my tamper wont fit below that. I guess this doesnt really have to be a problem if you dose so that you tamp above that level, but Ive found it annoying which is why im keenly awaiting my Synesso basket. I cant really comment on the Synesso as i havent used it yet but im sure Greg Pullman and many other people will have lotsa good things to say about it.
Hope that helps
All the best!