Question about video scalers

I recently purchased a Sony Bravia 40" lcd television and am less than thrilled at the quality of some standard definition material. Our previous CRT display, a 32" hdtv Hitachi Ultravision, actually looked better to me than the Sony and looked great on all programming, hdtv or standard. The Sony looked better than anything else in the store on hdtv, but the inhome experience lacks a bit on standard def. I'm sure a big part of it is that the larger screen magnifies source quality discrepancies. So, my question is, how well do video upscalers work and how much does one have to spend to get acceptable quality? I see everything from $300 units to $5k or more Runcos. Any tips or where to purchase and what to look for? FWIW, cable service delivered through an Atlanta Scientific Explorer 8300hd box/dvr connected via component inputs. I've got a HDMI cable on the way.