I was going to get an Airport extreme to use as my router and an Airport express to connect to my hr20-700. So I could just buy two Airport express and that would work instead.
And is the express as secure of a router as the extreme is?
Yes, using 2 airport expresses will work. What you won't have, however, is a "wired" LAN connection on the router - since the Airport Express only has one Ethernet jack, it hooks up to your cable modem.
If that's OK with you, then this configuration will be fine.
Having said that ... the current Airport Extreme is an 802.11n router, which means it's very high-speed. The Airport Express is "only" 802.11g. Not sure if/when an 802.11n version is coming. The Airport Extreme also has 3 gigabit Ethernet ports on it.
That may be something to think about.