If you have DSL, then you'll need a DSL modem. The WRT320N would work as your router, but you still need to have a DSL modem (either from your ISP or purchased by you separately). The WAG160N is a combo unit, it's BOTH a DSL modem and router.
So minimally you need to determine where your DSL modem is coming from, the ISP or is it your obligation to purchase one.
And btw, dual-band isn't really necessary unless you have a need to support 5GHz wireless clients. Most ppl only need the more popular 2.4GHz band and will consider 5GHz if the 2.4GHz band is too congested and causing excessive interference. Doesn't hurt to have it, but it's a waste a money if you don't really have the need.