If you're going to have plants, I definitely recommend fake ones as most African cichlids will eat and/or destroy live plants. Not to mention that the high pH and hardness will be tough on many species of plants. Personally, I like the more spartan look with big heaps of rock as your only decor. Let me find an example...
This one is interesting, with big, round rocks: http://www.malawimayhem.com/images/g...tin%20tank.JPG
Here's another: Malawi cichlid tank image by nicksweeting on Photobucket
As for filtration: I find myself making this recommendation a lot, but I really support it so here it goes again. I don't personally own an FX5, but I've heard bad things about them. First, the price: $300 is about what they run for. Next, you get the fact that the flow rate through this sucker is absolutely enormous. The problem here is that you don't get much contact time between the water flowing through the filter and the filter media. I've seen a lot of information (including a thread on this forum in the DIY section) about people messing with FX5's to increase the capacity of the filter media compartment to improve filtration. Were it me, I would instead spend that $300 on two Marineland Emperor 400 power filters and two Marineland Magnum 350 Pro canisters. This would give you a much higher total flow rate, better water movement throughout the tank, more total media space plus the added biological filtration you'd get with a total of four bio-wheels. Of course, you could substitute similar filters such as Aquaclear power filters for the Emperors or the smaller Fluval canisters for the Magnums.
Of course, if you're really good with DIY projects you might consider building a large sump type filter. These are really some of the best biological filters you can put on a fish tank as you can basically customize it to have huge amounts of contact time with biomedia. You might also want to add a couple of strong power filters set up for simple mechanical filtration. It would be a fun thing to do, but in the end might end up being more expensive (and certainly time-consuming) than just buying filters.
Just FYI, filters are a heck of a lot cheaper online than in brick-and-mortar stores.
About the fish: it's really best not to mix fish from the different lakes, so I suggest picking a lake and sticking with fish just from that one lake (the reasons for this are largely tied to incompatible personalities between fish from the diffferent lakes). If you want a variety of colors, Lake Malawi fish are probably the way to go. Lake Tanganyika has less brightly colored fish, but they come in a more interesting variety of body shapes and sizes and have unique behavior. It's really a matter of preference, but also be aware that Malawi fish are generally easier to find.