This Mac is only the old Airport, B standard. Its not that slow at all so don't get all worried that you think it will suck. It can go just as fast as the modem can go.
Pretty much any 802.11g PCI card with a Broadcom chipset (same one used in the extreme cards) will be recognised natively by the Airport drivers. Apple did this for compatibility for users with older equipment, just make sure your Airport software is up to date.
Your best bet is probably to look at cards made by Linksys or Belkin, though several other manufacturers use Broadcom chipsets also.