The 2G thing is a safeguard policy for all the telcos. As long as you are not doing illegal stuffs, you should be alright. In fact, if you burst the 2G limit, there will be people calling you to check whether are you really downloading that amount. Cause sometime it can be the work of some viruses spamming out mails. Which then you can know and do something about it.
THe speedwise... hmm...
My old card seemed to switch between the 3G and GPRS faster then the new usb modem thingy. But the new modem can handle my faster speed plan.
Lately, seemed like a lot of ppl taking up the plan. Like area jam up in the night. Think of how your handphone always cannot call out at the New Year celebration a suntec.
Hope that helps.