6 memory slots - 3x1gb and 3x2gb -- good or bad?

Hi,

I just ordered a Dell Studio XPS desktop with an Intel Core i7-920 processor.

The machine has 6 DDR3 DIMM slots and comes stocked with 6 x 1GB DIMMs = 6GB RAM.

I know that I need at least 8GB RAM and that the memory for this type of system is triple channel.

So, my question is: Will I take a performance hit by installing 3 x 2GB DIMMs and 3 x 1GB DIMMs? I.e., does the triple channel advantage only occur if ALL SIX slots use the same capacity DIMM, or can I enjoy the triple channel advantage if 3 slots use one capacity DIMM while the remaining 3 slots use a different capacity DIMM?

Thanks!