It sounds like you have up to $1500 to spend to improve your system but you might not NEED to spend it.
1. First, purchase an SPL meter at Radio Shack ($50) and an Avia calibration disc ($30-40) so you can setup your current equipment properly. This includes setting the size, distance, and (volume) level properly for each of your speakers. If you still aren't happy, pursue one or more of the upgrades below.
2. Upgrade the front L/C/R channels. Check out this deal on the RBH on wall speakers:
http://store.audioholics.com/section/66209/1/xs_pricelh/on-wall-speakers
They would look nice mounted on the sides and below your new display and provide a performance upgrade for the front channels. Some nice bookshelf speakers in the $200-400/pair price range would also be a good upgrade option. For $900-1000 you can get a good quality 5.1 speaker package.
3. Upgrade the sub: Check out the SVS and HSU models with 10"-12" woofers. You will get the LFE information down to 20 Hz which is lacking from your current sub.
4. DVD player: I would recommend one of the Oppo digital upscalling DVD players for $150-200. If you are ready for BluRay, the PS3 is $400.
5. Receiver?: I don't see any reason to upgrade the receiver unless you want to take advantage of the latest audio formats on BluRay movies. If you get BluRay, then receiver in the $4-500 price range like the Yamaha RX-V663 offer HDMI audio processing.