The high definition scientists over at HD Guru make a possibly obvious but nevertheless important point. You should not buy a DVD upscaler—though not for the reason you think. It's not because DVD upscaling doesn't bring some benefit to watching DVDs on HDTVs; it's because most TVs are upscaling already.
Not only can HD Guru not tell the difference between many standalone and TV upscalers, they point out that:
As seen in the HD Guru tests of 2008 HDTVs, the internal scaling circuits have been upgraded, compared to previous year’s models and are generally superior to the scalers within many upconverting DVD players.
Extrapolating that point, you could actually be doing yourself a disservice by using an old DVD upscaler with a fancy new television.
Needless to say, HD Guru recommends you stick with Blu-ray players if you've got a few hundred bucks burning a hole in your pocket. [HD Guru]