First wireless network: b/g sufficient? Connect to router?

About to install my first wireless network (!) to allow Internet access via laptop (Sony VAIO) or iPod Touch from 2nd floor of house (destkop computer on the 3rd floor is an old Dell Dimension 4600, running Windows XP Pro, SP 3).

Already have a multi-user DSL router (Zyxel Prestige 600 series) that connects two desktop computers (via wired Ethernet connection), but only for Internet access.

Question 1: I was thinking about buying an Apple Airport Extreme, so as to have 802.11 n capacities, but I have a very old set of Dell hardware lying around that goes through 802.11 g:

TrueMobile 2300 Wireless Broadband Router
TrueMobile 1300 USB Adapter

Is it worth trying the Dell stuff first, or will it be hopelessly slow?

Question 2: I really just want Internet access via the wireless network ... don't really need to network the computers and worry about security. The Dell instructions are to plug the Router into the computer. But could I also plug the Router into my multi-user DSL router?

Thanks.

Joel