Don't try adding a mini-PCI even if there's a slot for it as the computer may not accept anything but the one normally fitted at the factory.
That leaves your choices as USB or PCMCIA. Personally I favour the latter. Although they cost a little more PCMCIA seems more reliable and there's less sticking out from the computer to get damaged.
As for brand -- there's not much difference. Unbranded ones may not offer the same degree of after sales support as, say, Netgear, Linksys, Dlink.
Most new cards are 802.11n wireless -- but you should be happy with 802.11g.