I repeat. depends. If you wanna shape download speed, you must use your incoming bw. SInce download traffic is 'outgoing' from LAN nic, you can shape it.
hmm.. i would enter the 256KB (just guessing) kinda like Frame Relay CIR.
but then again just guessing. err.. whats the problem? assuming u have at least a 10MBit interface, you can still transfer data 40 times faster than the ADSL can send it. so the NIC itself is pretty much always sleeping... (when not processing millions of NetBIOS packets )