Where in relation is this router to the floors in your home?
I think the only possible reason you might be seeing these issues on the 2.4Ghz radio is due to interference for something or distance could be too far.
I don't think it would cost a great deal of money to wire up your home either.
One thing maybe you should review and research is maybe getting some D-Link Power Line products installed for those devices that don't need a lot of heavy bandwidth usage. Then if you can wire up those devices that do need heavier bandwidth load like downloading or gaming. I have had to wire up my existing house for LAN using these:
http://forums.dlink.com/index.php?topic=52726.msg202260#msg202260Again, these are not ideal however if you need a wire ran and out of the way, these work good.
I'm currently in the middle of a new addition to the existing house and ya, thats get all CAT6A installed. Hopefully soon.
I think you could even run a line on the outside wall from one floor to another and install a external Network switch and a additional AP up on the stop floor too extend range as well. I think there are some possibilities and still keep this router going for a long time. I still love my 4500. Works great man.
I am curious if you took the 4500 to a different location, say a friend or families place in same town, do you see this wireless problem?