Why isn't my iMac transferring files over network at wireless-n speeds?

First of all here's my setup: 30d/4u Charter internet coming in through my Linksys modem, from there to port 1 of my 2006 Intel Xserve running NAT and DHCP, out from port 2 of the Xserve into my AirPort Extreme running in Bridge Mode, and then out to my other devices.

My AirPort setup is as follows: Internet tab-DHCP, address set by Xserve; Wireless tab-Create a wireless network, WPA/WPA 2 Personal; Wireless Options...-5 GHz network name is not checked, Radio mode is set to 802.11a/n-802.11b/g/n (Automatic), both 2.4 and 5 GHz channels are set to Automatic; Network tab-Off (Bridge Mode).

I am transferring 100+ GB of data across the network to my Xserve, but it is only Xferring at 5.5 MB/s, or around 44 Mbps, which isn't even at wireless-g speeds.

My iMac is the Mid 2010 27" BTO w/ 2.93 i7 and 8 GB of RAM. It is also wireless-n compatible.

The Xserve is wired and they have had gigabit NICs since the G5s.

Things I've tried:

In Wireless Options... I tried turning Radio Mode to 802.11a/n-802.11b/g and leaving 5 GHz network name unchecked, but the speeds were down in the 2 MB/s range.

Again in Wireless Options... I checked the box for 5 GHz network name, and left Radio Mode in Auto, joined the 5 GHz network on my iMac, tried again, but again only 2 MB/s or less.

In Wireless Options... again, I checked the 5 GHz network name box, and set Radio Mode to a/n-b/g, but ocne again, it ran at 2 MB/s or less.

The AirPort Extreme is model A1354, which is obviously 802.11n compatible, and the iMac is also 802.11n compatible.

Any help would be greatly appreciated as this is painfully slow and I don't know what the bottleneck would be here.