KG, kung meron ka ng K750i and you would want a clamshell handphone (based on choosing Z520i),suggest ko sana W550i. Clamshell size sya pero rotary type.
If second fon mo lang sya, I guess camera resolution is secondary (but not VGA) and price should be cheaper than your K750i,why not go for a music fon? Ganda din ng Z530 and mayroon din syang walkman version, W300. Medyo mababa nga lang cam resolution ng dalawa * VGA.