EAX and A3D both work well with 2-speaker setups. In fact, they were developed initially so that people didn't need to rush out and buy surround sound for their PCs (a lot of good it has done, no?). Technically, you don't need either 4.1 OR 5.1 to effectively use either technology, because their main job is to emulate surround sound and the sounds coming from different directions. If you actually DO have surround sound, then the technology might work better because instead of emulating which direction a sound comes from, it actually comes from that direction. I'm not too versed on the A3D vs. EAX debate, but when it was important, I seem to remember that EAX was better for two speakers and A3D for four.
Bottom line, you don't need to worry about speaker setups to use either technology. It's a premium. 5.1 is rarely, and I mean *rarely* used in games. Maybe 1 or 2 out of every 50 use it, and only a few more than that effectively use surround sound--with the exception of games programmed for EAX and A3D. 5.1 is really only useful if you use your PC to watch DVD movies, and by the time you've just dropped enough for a decent DVD set-top box to watch movies on your TV, which would much more comfortable, most likely.
From what I've heard, the Klipsch Promedia 4.1 isn't limited by much, but the lack of a center channel probably isn't one of them. They rank higher than the majority of 5.1 systems, from what I've read. I plan on picking up a set of 4.2's when I get the cash later this year.
