encryption may be slowing you down on the router.
The router itself maybe stalling you because it can only filter packets so fast..
If you modem is pushing it straight to your pc faster than through your router, then contact you cable co and tell them this if you rent the modem from them.
Otherwise, find the vendor website and see if they have user forums or google your router + slow or your router + increasing speed or something like that.
Sounds like you are in happy land with the speed you are getting...wish mine were that fast or capable of.

My linksys has almost 9 computers on it during peak times. 2 laptops, 3 desktops, 3 servers and a voip box. I don't even hesitate in Aces High 2 when I play!

However at night when I get 'serious' about it, everything gets throttle to almost nothing and I'm in with the full pipe.
I have 24 ports punched down going into a switch which in turn goes via a single plug to the router. Pretty elite setup for a home loser. I may be getting a t1 in here for some webhosting and will need a 'real router' soon but until then, my old 54g is doing GREAT! I'm amazed actually.
EDIT: I did a quick search and came up to this discussions. Sounds like someone else having the same issue and it's gone unresolved. Do yourself a flavor and get a real router, Linksys (Cisco!)

I know that's not the answer but a quick swap from a best buy returnable or walmart returnable (better choice) would alleviate ANY doubts that it's the router right?
http://www.wirelessforums.org/alt-internet-wireless/wireless-router-slows-internet-speed-7666.html