Lag on WoW after setting up Network

I have a Amd 64bit 754 3200+ 512 ddr 400 mhz ram, 6600Gt. My girlfriend has the same except 939. Previously I played World Of Warcraft fine, very little lag. Oh I also have Cable Internet. After I build her computer I set up a Network on a Dlink Wired Router. Immediantly I noticed a difference on my computers capabilities, hers completely refused to play for more thatn 2 minutes without disconnecting. I should mention that we both had 1 gig of ram and hers ended up being bad (it is now being sent replaced in the mail by new.egg) so I had to lend her a stick of 512 from my Comp. Not sure if that should make so much of a difference to where it wouldnt even let her logg on. ANy suggestions?

 

If you go straight to the cable modem, does it cure the problem?

--This would make certain its the router.

What Dlink router is it?