Airport Express on separate 802.11n network?

That seems like a reasonable choice, although I haven't heard of the brand name. Here are some additional choices, that are claimed to be Mac compatible:

Rosewill

Edimax

EnGenius

Amped Wireless

I've been using an Amped Wireless USB 2.4 gHz adapter on one of my Windows machines with excellent results, communicating from my basement to a wireless access point on the second floor of my house. But their dual band product linked to above may be overkill for your purposes, considering that both ends of the link are within the same room. Note that its spec for maximum transmitted power corresponds to 400 milliwatts!

Using a wireless-N router, connected to the Mini's ethernet port, would also be a good approach, but probably more expensive than most (or all) of these choices.

Best regards,
-- Al

Thank you Al. I appreciate the time you spent to help me. I ordered Edimax from your Newegg link on special for $19.19 (offer I could not refuse). It should work since distance is only 15' across the room, but in case it doesn't I have your link to Amped (yes, power is huge and antenna is directional).

Edimax comes with Windows drivers only (same on the website). Manufacturer claims only Windows compatibility (shame on you Newegg). Luckily I was able to find Ralitek chipset number after opening the case and to download driver plus utility program.

I can create two separate infrastructure networks but they don't work at the same time. Peer-to-peer network in addition to infrastructure network would perhaps work but Apple Express won't work in that mode. I ended up disabling Mac's radio (called Airport) using Edimax instead (one network as before). I set Edimax to broadcast on 2.4GHz and 5GHz simultaneously. Router is 2.4GHz only but Airport Express listens on both frequencies. I tested it for two days and have no single dropout even with microwave on. It might be addition of 5GHz or Edimax transmitter is just a bit stronger. Keep my fingers crossed.