The Cube runs 10.5.8 the last supported upgrade for G4.
Thanks, I'll have to update my notes which only reflect 10.4.11 as the latest with the native processor.
The Airport card have firmware 9.52, I think it's the latest and it's an Apple card. The S/N PW505P1QAC AirPort ID 003065B741D.
When I was connected with the Airport Express I used WPA/WPA2 in the router and in the network menu it signed up as a WPA2 connection!
Yes, that would be a revision 2 card. However, again, it will only support up to WPA, not WPA2.
The AirPort Express run version 7.6.8 and 802.11n (802.11b/g compatible).
... and that shouldn't be an issue for the AirPort card in your Cube.
The Time Capsule are a 2TB tower version run version 7.7.8 and 2.4 GHz auto/ 5GHz auto. I can't find anything about 802.11b in the settings!
... and you won't find them or any other Radio Mode settings for the 802.11ac models. They were removed for these models in the AirPort Utility. WEP is no longer supported either with these models.
In theory, your Cube should be able to connect with wireless security set to: WPA/WPA2 Personal using TKIP ... or no security set at all. Without seeing a packet trace of the wireless 802.11 management traffic it will be difficult to understand what is going on or not that should.
Typically, when a device can't connect even with wireless security enabled, it would point to the wireless card as the culprit.