yes you will see a diff
Mr. I-can't-spell,-punctuate,-capitalise-or-give-good-advice doesn't know what he is talking about.
It doesn't matter what slots you put them in. Placing RAM faster than your existing RAM alongside the slow stuff results in ALL the RAM operating at the slow speed. (This is, by definition, is what "By SPD" does in the BIOS).
Additionally, it is likely overall memory performance will be slower than it was: using all four memory slots for "Dual Channel" ("Duel" = a fight amongst two people. "Dual" = "two") generally results in increased overhead for memory management.
Mr. Sirheck is also delusional on a second point: you can stuff the new memory straight into the spare slots. Making sure the machine is turned off first. Given your dumbness, it would benefit you to examine the installation of memory before trying to use a crowbar to fit it. DO YOUR RESEARCH!
Why were you suddenly so desperate to get more memory? Were you consistently using 1GB and paging a ton more? Personally, I seriously doubt it. I suspect someone saw you "only had 1GB of memory" and told you that you absolutely "HAVE TO HAVE 2 GB!"
Remember golden rule of RAM: Unused Physical Memory might as well be installed in the top drawer of your desk for all the good it does you.