Re: 2.4 Ghz network cards with a higher frequency router?
All wireless routers work at 2.4Ghz, you can't have difference freq like phones for this. You can choose a different CHANNELS for your router from your 2.4ghz phone (it will help but you will loose connection whenever the phone will be in use).
For Dlink XtremeG:
If you enable xtremeG it uses channel 6 ONLY which reduces compatibility. Put xtremeG as optional (or disable, doen'st make much difference in speed as compare to regular G speed).
I believe there is an option to enable xtended support (forgot whats its exactly called). Might need latest firmware to have that option. What this does it actually reduces the speed your wireless is connected on, which boosts the signal strength and it keeps the connection alive and slow (rather than break it altogether).
Linksys is the only router that has an option to "boost" signal strength option that you can check on/off. Which suppose to help.
You restteing after couple of days problem lies the way you configured your rotuer (mainly ports settings etc). Reset to default for now and see if it occurs. If not then re-do your ports settings but becareful on to have conflicts etc otherwise router will be "confused" and will require reset.
Hope this helps