I'm in a similar situation, with erratic data usage every month from none to a week or more. I had the Verizon EVDO card but didn't renew my contract wasn't worth $80/mo for my usage. Verizon has a deal where you can pay $15/day rather than the monthly fee but they won't do it for the data cards, only laptops with built-in EVDO modems. I was just trying a Cingular Option GT Max card but I'm sending it back because of poor customer service. That's another story.
If you aren't in a 3G area, you can get a tethering plan for the BB that will let you use it as a modem for an additional $30/mo.
If you get the EVDO capable BB from Verizon, they charge only $15/mo for tethering and you'll get EVDO (400k-700k) speed. I think that's a really good deal considering the network coverage of EVDO (160? cities or more). If that doesn't work, I'd check the Verizon site and see if you're in an EVDO coverage area and get that if you can, it is the same price ($60/mo) as Cingular but the speed is 400k-700k with EDGE being about 80k-120k. Cingular hasn't released a rollout schedule that I've seen but you can try www.howardforums.com
there are some threads about the new 3g rollout.