What kind of tower do I need?

Hello,
I'm looking to get a new tower, and have what is; originally a HP Pavilion a1230n, 2.40 gigahertz AMD Athlon 64, 3GB pc3200 ddr ram, a 512mb GeForce 9600 GT OC graphics card, with what is stated as a 'ASUSTek Computer INC. Amberine M 1.03' motherboard. I've read about these BTX towers meant to replace the ATX etc. towers and were being adopted by several manufacturers, but I really dont know what kind of friggen tower I really need. I'm kookin' to upgrade with a new tower. ATX? Mid? Mini? I cant figure out what fits right before I buy it anywhere online. Figured I'd ask!