Assuming it has PCI or USB socket free any wireless adapter will do (though I prefer PCI).
The trick is to find one with a generic driver for the chipset used. Usually these are found on the chipset manufacturer's sites like Atheros, Broadcomm etc.
The Linux types are pretty smart at listing which models use which chipset so get looking on Google.
As far as brand goes, doesn't especially matter as (per above) they all use a small pool of chipsets -- the main difference is in support which should be better from the likes of Dlink, Netgear, Linksys than from a no-name brand. But as few, if any, support Linux this may not be an issue for you.