First, just in case, check with your cable provider as to when they will be doing HDTV. It will probably be two to three years but at least you asked.
Then find out what's available OTA in your area. The titantv.com site has program listings for DTV which show which programs are in HD - you have to register but it's free. And go look in the avsforum.com local HDTV section - you should find posts for your area and if you don't then just post a questions and you should get lots of help. If you are near Boston there are several threads.
The networks are really only available OTA - Dish has CBS in HD but you have to be in an area served by a CBS owned and operated station to get it.
On satellite both Dish and Direct tv have HBO and Showtime in HD. Only Dish has Discovery HD, and only Direct TV has HDNet which shows nothing but HDTV. HDNet is adding two more channels sometime in the future. If the merger goes through then I assume the merged company will offer all these channels - eventually.

There are several Direct TV plus OTA receivers (set top boxes or STBs). Right now most of the manufacturers are bringing out new receivers so the old ones can be inexpensive by comparison. RCA, Hughes (and the Toshiba and Mitsubishi "clones" built by Hughes) and Zenith all have their supporters.
Many people say Zenith is the best from a sensitivity and multipath rejection perspective. The Hughes STBs have a really clever Advanced Program Guide which allows you to integrate OTA and Direct TV stations into a single guide based on your zip code (but not the cable guide except in a very few cases). You can get the Hughes E-86 right now for around $450 (I think the elliptical dish is extra). The RCA is the oldest but is often recommended (although you will probably need a special connector called a transcoder to connect it to your TV). If you decide just to go with OTA then Samsung make an STB which is supposedly very good.
If you decide to go with the Dish network then they also make an HDTV STB which with the correct module will do OTA as well. I don't have Dish so am not sure of the exact model numbers.

There is a new generation of Direct TV STBs which will probably become available late this year. These don't seem to offer much over the current models with a couple of exceptions:
Current STBs in general don't have the HDTV and the regular s-video outputs "live" at the same time. This means you can't watch a program in HD and record it in SD at the same time. Some of the new STBs have this capability.
Current STB s are not set up to handle the digital copy-protection technology which is coming (maybe!). But since your TV does not have this either then it may not be a problem. If you look in the AVS forum you will find many threads about copy protection, "fair use" and the possibility that copy protection might prevent people with today's equipment watching certain HDTV broadcasts.
It's possible the new models may have better multipath rejection which might be a real advantage if your location suffers from this problem.

If the Dish-Direct TV merger goes through there will probably be a completely new set of equipment but as part of the merger "terms" everyone will apparently get replacement systems of some type - again this is likely to be a number of years away.

FYI I have the Hughes E-86 with Direct TV and OTA. Here in Dallas we are lucky to have HDTV OTA from several stations and it is quite spectacular.