Ah yes . The upgrade. We all have been there and I am currently still going thru it. Only been 6 months now....... Happy happy joy joy.
Most people will tell you lights and skimmer then flow.
You need a good light setup to keep most clams. MH lights etc. A decent skimmer will save some headaches down the road too. Everyones oppinions very as well, so do some research.
The good thing is you have come to the right site. Many people here are helpful and have great used stuff for sale. So that is where I would start. Make a list of what you have and what you want in your tank and then what you need. Then get a second job
HTH Cheers