$1500-2000 can be made to work for a sub, receiver and DVD player. You mentioned ceramic tile floors so you may need to reserve a little bit for acoustic treatments.
I would probably prefer to go with at least 100wpc for the Polk's. Most of your receivers with the latest features will give you enough power. There are plenty to choose from, but I'll throw out a fairly popular choice in the
Onkyo TX-SR805 7.1 Channel Home Theater Receiver for $730 shipped. You may find it cheaper... make sure you have a valid warranty.
For a sub... $600 for a
SVS PB12-NSD... which should be sufficient for now, but maybe consider adding another later on. You can test it and see.
That's $1330 right there. If you just want a good upconverting DVD player then shop around for a Toshiba HD-DVD player for $100-150. If you want Blu-ray, it will cost you at least $325 unless you buy used.
That saves you a little bit for the surrounds and maybe some acoustic treatments.
A DLP will be quite a bit cheaper than a plasma, especially at 50" or larger. If you plan on spending that kind of money... consider a projector, which will increase your screen size and should increase your viewing experience.