I am using a Westell 7500 wireless-G DSL router. Would using dual-band-wireless-capable equipment with my router be overkill, or would there still be advantages?
If you're only looking to see if there are advantages to having dual-band equipment on an 802.11g network, the answer is "No".
Consumer-grade 802.11 specifications generally run in one or two RF spectra - 2.4 GHz or 5 GHz. Each also has its own speed limitations.
*802.11n is capable of multi-streaming with up to four streams, for a theoretical upper limit of 288.8/600 Mbps.
If you are only connecting to 802.11g networks, there is no benefit to using an 802.11n adapter - it will only be able to use the 2.4 GHz frequencies and throttle itself down to the 802.11g speeds, while on those networks.
However, 802.11n devices are so ubiquitous these days that there's really not much benefit (even in price) to not purchasing one either. This will open up your options for connecting to other, newer networks at higher data speeds - or, for upgrading your own network in the future.
One more distinction that should be made here is that not all 802.11n equipment is dual-band capable. Most of the inexpensive routers, access points, bridges and adapters only operate on 2.4 GHz. If 2.4 GHz (802.11b/g and most 802.11n networks) is all you're going to connect to, this is not a big deal.
But, if you want to be able to connect to 5 GHz networks, or upgrade your own network to 5 GHz in the future, you'll need dual-band equipment. The difference in price between single- and dual-band 802.11n adapters is around $20-30.
In the end, the decision will lie with you and will depend on your needs and available resources.
The dual-band (802.11n-draft) equipment will throttle itself back to speak to the router at 802.11g speeds, so there will be no benefit in the short term.
In the long term, when your router packs up and you go out and buy a new one (that is now 802.11n-draft capable), you will get the benefits of it without having to upgrade other equipment.
You could think of it as a 'phased switchover' between your 802.11g and new 802.11n-draft network.