Gee, you could make this into a semester class...
OK, MIMO stands for Multiple Input - Multiple Output, and is used to describe one of the key 802.11n technologies because even the acronym for any kind of real description is about a half a page long. What they are doing is using some very tricky space-time coding to be able to send and receive multiple data streams in the same frequency spectrum at the same time and at virtually the same power level, and rather than interfere with each other destructively they interfere CONstructively and result in near double (or whatever multiple is used) effective throughput. To make it work, the actual analog radio transmitter stage must be duplicated for each of the "multiple" signals, and likewise the receiver. In addition, to function properly the two need to use antennas that are spaced some distance apart - opinions vary on whether 1/2 wavelength or 1/4 is sufficient. 1/4 Wavelength at 2.4Ghz is about 1.23 inches (31.25mm for you in parts of the world where distance measurements are not encrypted by ancient varied measures related to sundry bits of human anatomy.) Which is why the little USB "finger" adapters usually don't work all that well. A 2x2 MIMO means there are two transmitters (or "transmit radio chains") and two receivers. The third digit, if present, is the number of receiving antennas. There are 3X3 and 4X4 systems out there, with roughly 150Mbs added per "X", bringing the throughput up to the full 600Mbps promised (well, promised in the marketing sense) by the 802.11n standard. The coding involved is pretty tricky and works kind like having your wife yell at you at a football game. You can pick out her voice and understand what she is saying pretty well even with the crowd noise because you know what voice you are listening for and have a good idea of what she is going to say even before she says it. (Yes, dear, hotdog no onions and a diet soda..) Of course multiplying the radios also multiplies the cost and power consumption as well as the package size, so 2x2 is by far the most common configuration today.
As for your card, if it actually is a/b/g/draft n then it is a dual band card - 802.11a uses only the 5Ghz band, while b & g use only the 2.4Ghz band, so there would be no reason NOT to do n on both bands. I seriously doubt it is MIMO equipped, though.
Attenuation, that is signal loss through various means including absorption, scattering, refraction, reflection, and diffusion, generally affects higher frequencies more than lower ones. The 5Ghz band is much higher, and as such is more heavily attenuated by most (but not all) materials than 2.4Ghz signals. This is one reason that 802.11a never really took off for indoor use. 802.11n has a lot of tricks that allow it to use signals that would have been considered interference in older standards, bringing it's propagation characteristics on the 5Ghz band close to those of 802.11g on the 2.4Ghz band. It does this mostly by using reflected and refracted signals. There isn't much you can do about absorbed signals, and unfortunately that is what most interior walls do in that band.
As for the router-as-USB-storage-server, I use an Apple Airport Extreme at home which is a very capable dual-band n router with USB disk and print server capability, and it easily keeps up with streaming for me. I use it for Time Machine backups on four systems, plus about 500M of media files (mostly music, but some videos and podcasts) as well as a shared laser printer for the house. I don't do much HD, but so far it hasn't seemed to break a sweat even doing multiple operations at once. The technology is not new, particularly in the Linux/BSD world, and today's processors spend most of their time just looking for something to keep them from being bored. There are always glitches in new stuff, but I wouldn't be too hesitant about getting one of the "microserver" routers from any reputable manufacturer.
Power Line networking isn't new either, and it basically is what it sounds like - it modulates an Ethernet signal to run over a form of unshielded untwisted pair - in the form of the AC power wiring in your home. Very similar technologies are available to run Ethernet over various other wiring systems that may be present in your home, such as phone or TV antenna cables. Powerline may be a "wall wart" type brick with an ethernet jack in the bottom or side, or may be a very router-ish looking tabletop unit, some with several Ethernet ports. Most are simple bridges. There are more than a few standards out there and many variations between manufacturers on their "interpretation" of the standards, so if you do go this way I'd recommend sticking with one series of related products from one manufacturer. I've used these with good success to get signals to basements and garages well out of wireless range. It helps if you can get the units on the same "phase" of the power system (home power wiring has two "hot" wires at the breaker box, each representing a "phase", as well as a "Common" or Neutral wire and a safety ground). It doesn't go through power transformers, but if your house shares a transformer with the neighbors there is a chance that your signal will reach them too, so encryption is a nice feature. Some of the "A/V" units can sustain better than 100Mbps throughput, easily keeping up with most traffic and it's much easier to install than stringing Cat5 cables. You can find NewEgg's collection here.
You usually connect to a Powerline network device with an Ethernet cable (although some have a wireless Access Point built in for extending wireless networks) so they are best suited to desktops, game stations, media centers, and the like.
802.3 is the standards working group responsible for Ethernet-like protocols. 802.3u is the 100Mbps Fast Ethernet standard.