Will I see a difference 480i - vs - better?

Yes.

Progressive scan makes a big difference in picture quality. I am not convinced that upscaling DVD players are worth the money. I would wait for high def DVD players to drop in price before I bought an upscaler.

Man I bought the previous version a month ago and...it's amazing! There is a big different when you have your old dvd player connected with component and this new dvpns75h connected in hdmi to compare. Look at the following links to read about the previous model that is no longer available:
http://www.amazon.com/gp/product/B000A6TN9I/102-7351117-4084916?v=glance&n=172282

http://reviews.cnet.com/Sony_DVP_NS70H_DVD_player/4505-6463_7-31484375.html

http://www.circuitcity.com/rpsm/oid/131360/originURLEncoded/http%253A%252F%252Fwww.circuitcity.com%252Frpsm%252FcatOid%252F-12873%252FN%252F20012866%252B20012872%252B20012873%252Flink%252Fref%252Frpem%252Fccd%252Fcategorylist.do/rpem/ccd/productDetailReview.do#tabs

Anyways the next best thing would be to wait till blu-ray is available, but even then the first players will cost $999 in august. Bestbuy is already taking preorders:

http://www.bestbuy.com/site/olspage.jsp?skuId=7763823&type=product&productCategoryId=cat03013&id=1142288677966

I suggest getting the dvpns75h, it shouldn't cost you no more than $150, and it is honestly better than the other upconversion players.

As far as the dvpns75h, I bought it too (picked it up this week at bestbuy) to see if there was any kind of difference from the dvpns70h model, well there are two new picture sharpness options (mnr and bnr) and the size of the player is thinner.

You would be very happy with the dvpns75h, I've watched all my dvds over again because they all look like HD quality now.