What to get next?

[Andromeda18_]

I agree with -spike-, 'Now and Then, Here and There' and 'GTO' are two must have collections. I also recommend Haibane Renmei, Escaflowne, Ranma 1/2 and DNA2. Two series I think every anime collector should have are Last Exile (best anime I've ever watched) and Witch Hunter Robin. If you prefer HK then you're going to have to wait some months for Last Exile since the last R1 will be for sale only in December. The Witch Hunter Robin HK however should be out around September or October (2/3 moths after the last r1).
Two cheap R1 perfect collections I like are Steel Angel Kurumi and Soul Hunter (Hoshin Engi).