Dropbox?
I think Stefan's question is because it's too slow accessing files from the shared usb drive on his router. I have a Netgear router and have also tried this and it is pretty slow but still an order of magnitude faster than uploading files to the Dropbox server then downloading them again!
I would go for a NAS, I've used a friends Synology NAS and it's awesome in every way I would want it to be
Also I love Dropbox, just not for this
