All Xbox 360 Bundles at Dell.com are 20% off, just add one to your cart and then:
Use 15% off coupon code CTV2$2HX963145
Then use the 5% off stackable coupon code TP5NV0VMFJ9BCZ (expires Dec. 10, 2008)
There is also free shipping for each bundle.
Prices after discount:
Xbox 360 Arcade -...
The Wifi Adapter is $69.99 (used) on Gamestop.com, and $82.96 (new) on Amazon.com
I think its really worth the money to buy the adapter, for the tech packed into that thing its fairly decently priced.
Good luck with the decision making.