The HD box won't improve the SD quality by very much.. SD is 480 and very poor quality to begin with, it's just more evident now that we have higher quality displays.
An expensive video upscaler (Silicon Optix Reon (
http://www.siliconoptix.com/products...X/Features.cfm) for example) might be able to do a better job of it, but I've not seen a comparison in terms of SD television.
The receiver I want to get, the Onkyo 875, actually has that chip built into it.