Mobile broadband recommendations?

The major issue here is download caps. For most providers the cap is 5GB/mo and any overage is really expensive. She'll be okay with very moderate browsing, but Youtube or music streaming is right out. Bus speed differences between USB and ExpressCard will not affect her. External antennas are nice, but will probably only provide an edge in really sketchy coverage situations. HSDPA (better yet, HSDPA+) is quite a bit faster than EVDO Rev A, but Verizon does generally have very good coverage.

I can recommend the Verizon Mi-Fi, if you absolutely must be mobile. It's handy because it will turn 3G coverage into WiFi for up to five devices simultaneously, rather than a USB-dongle which only works with one device at a time. (and that device needs to have a USB port). About $60/month for 5 GB of data, which should be fine if Mom is a light user. Don't remember what the initial buy-in is for the unit itself. My parents moved to a remote area of NYS that doesn't have access to anything but dial-up, so they got a MiFi. Works well for their needs, but I have to remember to "tread lightly" when I visit.

I use http://www.millenicom.com/nmb.htm and highly recommend them: $60/month for 10GB, or unlimited data for $70/month. They resell Verizon/Sprint, I think. Super-responsive every time I've had to contact them (which is rare). I have the Pantech USB card which is on the Verizon network and *love* it. We've been traveling the country while working and it's really come in handy, very few spots without connection. They also resell http://cradlepoint.com/ routers - we can plug our USB key into the router (and wired Internet or a laptop that picks up wireless) and serve to multiple computers.

I kind of doubt she's in their coverage area, but I have been happy with the CLEAR 4G service. No caps and reasonably affordable (I'm still on their $30/month promo). Sprint also sells the service, and a combo 3G/4G dongle (you get 4G service when you're in a CLEAR coverage area, and the 3G when you're outside CLEAR range but in a Sprint coverage area -- the Sprint part is subject to a 4G cap).

(The Sprint part is subject to a 5GB cap, I mean.)

My father is on Cricket pay-as-you-go. It's $30/month, if memory serves, without a contract (though there's a cost to reactivate it if you let service lapse, and they have pretty draconian rules about paying on time). It has a 5gb cap, but since he only uses it to check email faster, it works for him. I agree with everyone else that streaming music/video is right out.

We actually stream music & watch YouTube through our mobile broadband card (have been for the past week and a half, anyway). I once watched Hulu from a moving bus with it. Depending on what else you're doing and how good your signal is, it's not like FIOS or a T1 or even a cable modem, but it does work. Would recommend a cap-less plan if you do it a lot, though. ;)

Thanks for the advice, everyone!

I ended up recommending the AT&T USBConnect Turbo. AT&T because my mom said she gets 5 bars on their 3G network, but had poor reception at her home three years ago with Verizon; Turbo over Mercury and the other because my searching suggests it's the most recent; and Turbo over the other recent one, Velocity, because the Velocity has GPS which she really wouldn't need (on the theory that extra, unused features are somehow bad?). Not the most definitive answer, but that's what I ended up with. Thought I'd add that for anyone who might find this useful down the road.