Your Sony should work just fine. If your wanting to buy a 1080p DVD player, you're talking about a Blu-ray or HD DVD player...You'll likely be spending a bit more cash on one of those than an upconverting player.
I'm planning on buying a 1080p LCD TV soon and realized I need to upgrade my DVD player as well. I picked up the Sony 5-disc upconverting player but noticed on the box it upconverts to 1080i. Will this player work with the TV I hope to buy or will I need to buy a 1080p player? If so, are there any 1080p players currently on the market that hold more than a single disc at a time?