I expected excellent 11b performance because the WPC55AG is essentially the same as the NETGEAR WAG511 [reviewed here] with both products being clones of the same Atheros reference design. But it takes both client and AP to make a good wireless connection, and it seems that the Broadcom radio isn't up to the task.
Since the client application for the WPC55AG neither works on WinXP, nor allows you to control the operating mode(s) of the card, I switched to a Linksys WPC11 ver 3 802.11b card to check out the 55AG's 11b performance.
Figure 12: 11b throughput with Linksys WPC11 v3
(click on the image for a full-sized view)
Figure 12 shows low, but relatively consistent average throughput at all four test locations. But it also shows how average numbers can mask high throughput variation, which it appears the 55AG's Broadcom-based b/g radio has plenty of!