If your performance hit is due to network interference, then upgrading your internet connection speeds will do nothing.
Think of your wireless router as a pipe that connects your network to the internet. Normally that pipe is far larger than anything coming in from(or going out to) the internet, so there is alot of extra room in the pipe. If your network has a large amount of noise/interference it will reduce your network capacity, so your networks pipe is smaller than what is coming in from the intenet, so it doesn't matter how much faster that internet is, it doesn't change the fact that your network can't handle more traffic.
If you are sure this is the issue (ie it fixed itself if you are beside the router), then the fix is to get an access point and hardwire it to the floor where you are having issues with the wireless signal.
I don't have any personal experience with them, but you would be looking for something like
these.
If you don't have a cat5 cable running to that level, then you can do a pull of Cat5, or use
Power Line Networking solution. Again, I don't have any experiece with these, but if you shop around you can compare prices/quality to determine the best one for you.
Edit: Actually I just noticed this
powerline wireless access point, but it seems really expensive