There really is no "best" amount. Some people say always buy in for the max, others like to do shorter buy ins. Generally people who are better post flop are going to like a deeper buy in, because that will maximize their strengths post flop. Since you play a lot of sit and goes, you might be more comfortable initially buying in for $100 and using your experience with push/fold strategy, rather than buying in for the max, where you have more post flop decisions.

Dave

I recommend having about $500 available, and either keeping your stack at 2-300 while you're playing. But it all depends on your comfort level. The last thing you want to do is play too scared because you have too much money on the table.

I wrote an article about this very subject for AllVegasPoker a while back. I think it would be very helpful to you if you have time to read it. Good luck at the tables with your transition to the wonderful world of cash games.
http://www.pokeratlas.com/table-talk/announcements/las-vegas-news-the-short-buy-avp-strategy

@BentonBlakeman Pretty much everything Benton says, I concur. So highly recommend reading his article(s)

Once you feel comfortable at a particular table,its recommended to optimise your opportunities to relieve your opponents of their stacks. This entails increasing your stack to the maximum. This doesn't apply if other players are holding minimum chips. In that situation,Id be reevaluating my choice of table,and moving on. Another plus,is that your larger stack helps keep the bullies at bay. They generally prefer to exploit smaller stacks held by passive players.