Does anyone rember how to calculate impedance?

Gregadd,
Your question is rather open-ended as you simply say that you are putting speakers in parallel & speakers in series. You do not mention anything about the type of speaker being used.
As you might remember, speaker impedance is freq. dependent. What are called "nominal 8 ohms" speakers are 8 Ohms over a very small range.
Further, what are the resp. efficiencies of 2 speakers i.e. what is their dB SPL measured @ 1m for 1W input? Depending on this, the 2 speakers demands on the amplifier could be very different. The one that is more inefficient could suck up all the juice from the amp leaving the more efficient one clamoring for power. This'll probably show up as unequal SPL of music for a volume knob setting.
The speaker x-overs will play a dramatic role in how they load or do not load the amplifier output impedance. This will make the effective speaker load difficult to calculate @ the amp end.
3rdly, your speaker wire will also figure into this equation. Using a cable that is highly capacitative could send your amp into oscillations (unless the manuf. ensures stability under highly capacitative loads). Using a highly resistive cable will kill your amp's damping factor & wreck the bass. This'll show up as flubby bass & as well as inadequate bass. In adequate bass will cause you to crank up the volume *possibly* furthering amplifier thermal runaway.
I would suggest that you use 2 speakers are identical in efficiency, variation of load impedance & type of x-over used, if possible. If not possible then try to use speakers are nearly identical so that the amp can service both speakers just as well. If neither is possible then ensure that you have a honker of an amp that can put out gobs of current to drive virtually any speaker & be stable at the same time!
FWIW. IMHO.

Foxtrot-Parallel is with both speaker wires connected to the amps speaker terminals. Series means one speaker lead to one speaker, Then you run anther wire off that speaker to the other speaker.
Bombaywalla-You guys never cease to amaze me with the depth of your knowledge. See the thread on "what makes my reciever hum." I gave this guy some advice and wanted to be sure I did not make his receiver blow-up.

He was essentially trying to run an outboard woofer without the benifit of an outboard crossover/ amp. I would purchase a multizone amp if I were trying to run mutliple speakers. They also have receivers that allow you to run two pair of speakers at the same time.

So Gregadd, you're telling me that you either 1) don't trust your hearing, or 2) worry about what you can't hear.

I trust only my ears! Evaluating components is an art. Designing and building them is a science.
Since no component system or room is perfect once I've identified a problem, how do I correct it? Do I bring home every product and insert it in my sysytem? Suppose I want more bass. Do I get better speakers or do I make room improvements? Measurements can't take you all the way home, but they can eliminate the pretenders.
It can also save you money. Many products use expensive overkill to solve problems. Big is good then bigger must better. A few measurements could tell you at a certain point no signifcant improvement would occur. It's no doubt in my mind that many cables are using exotic materials when it would better to install better RFI Shielding and better connectors. Most cable companies don't even supply basic measuremnts like capictance or the cables ability to reject radio waves. Maybe they should do some measuring before they charge us megabucks for upgrading to platinum.

One of my fun things to do is look at systems on audiogon. I can't belive the horrible speaker placement. Speakers so far apart they couldn't possibly be getting a proper stereo image. Cables that cost more than thier components, dipole speakers pushed up against the wall. Almost every Martin Logan I saw was too close to the wall. ( I only hope some of these were posed for sake of the picture).

We have to pay some attention to the measurements.Let's not even mention how some audiophiles idea of "good sound" is ludicrous.