Alan,
A wireless network works at the speed of the slowest connected device, ie your 11b DWL-520.
Also, 'speed booster' cards typically only work with the same make of router, eg a network with a Linksys WRT54GS & 2 Linksys GS cards will operate at upto 108Mbs, but a Dlink speedbooster would probably only work at standard 54G.
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As for encryption, WEP is not very secure at all. Nor is MAC filtering. A competent wardriver could read any 'non-https' traffic on your network within a short time.
If possible, I would look to upgrade the 98 system asap and switch to WPA or WPA2 security.