Before I installed any new hardware, I'd toss in a Linux Live CD [many Linux installation CDs let you boot Linux right off the CD and try it out without altering your computer in any way] and look at the connection speed and run some speed tests.
Linux Live CDs are great ways of removing software or drivers from the equation entirely. If you get the same results, my next step would be to check the BIOS settings and also check Asus' site for a BIOS update.
If you have the latest BIOS and verify the BIOS settings are correct, and you still experience 10 Mbps ethernet, then by all means toss in a PCI network card.