So if I buy the single cpu dual core, I should get at least the same or better performance than a dual cpu, right?
Also, will it show two CPU's in task manager or one?
1) Yes (unless the dual CPU were both dual core)
I don't think this is totally true. I think the cores share cache or bus or something. There could be contention for anything that the cores share. I'd double check. Two cores should be better than one, but not equivelant to dual cpu.