It boils down to the processing capabilities of the chip on the DVD player. Forget the resolution. I can give you an $80.00 Samsung DVD player - if you think it's going to look as good as your Pioneer, then you're crazy! Many people are suckered by resolutions and upconversion. DO NOT BUY INTO IT!
What you want, above everything else, is a quality product, and you already have it with the Pioneer. Spending a fair bit of money on a DVD player upgrade doesn't make much sense in my opinion for what will likely be perceived as no image improvement at all.