Assuming that you got the case right (IOW, you're getting a transfer rate of 9-12MB(tytes)/second, not 9-12Mb(its)/second), you're basically maxing out the connection - 12MB/s = 96Mb/s.
If you want faster, you're going to need to upgrade to a Gigabit network. For the NICs, I'd highly recommend that you get Intel cards - the chipsets (and drivers) are rock-solid stable. Worth the extra few dollars that they cost.
Namely, I'd recommend this card
if you need PCI, (this version
if you need a low-profile backplate), and this card
for PCIe (comes with both full- and half-height brackets.)
Note to you: these are OEM-package cards. You'll want to download the driver package from Intel (no disc in the "box.")
But, you don't necessarily need to replace the router (but, this is a good excuse to upgrade.....) - you could also buy a simple 10/100/1000 switch, and connect all of the PCs to it (with the switch then connected to the router.) An 8-port Gb switch can be had for ~$65; or, a newer router (like, say, a DIR-655) is <$100 (varies slightly, depending on sales.)
Advantage to switch: you get 7 Gb ports. Advantage to router: only one box (but, only 4 ports), and you also get an upgrade to an N-capable wireless network.
Oh, and: since nothing's broke here, and it's more of a general "how do I?" network question, I'm going to move this to Network Nook.