I don't think iBurst will swap your iBurst desktop modem for a USB/PCMCIA modem. But you can phone their call center, and ask them.
Alternatively, ask around on these forums. I'm sure you'll find someone here willing to swap with you. Then phone iBurst and let them know, as they will have to update it on their systems.
I recommend the USB modem, for maximum portability. Use it on both desktop and laptop. Take the internet with you wherever you go.
I'm not sure if you can connect the USB modem to an external antenna, but even if it is possible, it will be much more cumbersome/inconvenient. I know that iBurst performs pretty decently when connected to an external antenna. Don't know how well the USB modem will perform without an external antenna.
If you are looking for true mobility (which I suspect is the reason you are switching to a laptop), with decent coverage wherever you go, then iBurst shouldn't be your first choice, but rather Vodacom 3G/HSDPA, followed by MTN. They have the best wireless coverage in the country. Most brands of laptop come with models that have a built-in 3G/HSDPA modem. I'd go for one of these. It's very convenient. You don't have to worry about carrying an extra USB modem, or losing the simcard, or both. Everything is hiddent away. Also, I know the Acer laptops come with a very cool, slide out antenna to improve signal. It slides out when you need it, and slides back in, hidden away, when you dont.
