If I had to replace my Pioneer deck, i'd run out and try the following:
Sony DVP-NS70H (only one minor inconvenience of a 2-pixel wide bar at the very top of the screen in full-screen mode)
or,
Philips DVP5960/37
I say "try", since most retailers have a return policy of 7-14 days and should give you enough time to test it out to make sure you enjoy it. Remember, all upscaling really does is make the image look a bit sharper in higher resolution, its still a low-grade DVD with the annoyances of compression and jaggies.
Component is the best analog signal available, and having the best component cables will give you the best analog output if you're not using HDMI. I use it on my DVD player to my TV and the playback is incredible on my Pioneer 578a deck.
If you want to do upscaling with your hdmi-upscaling dvd player, you'll need to have an HDMI connection anyway. HDMI is better since its pure digital, and makes the biggest improvement on colour output and a rock-solid stable image on the screen. If you're moving up from s-video, its like night and day.
Not at all. 20-25 feet is a typical "long" cable length and HDMI and DVI can both go that long without loss of signal (glitches). There are cables that can even go up to 50 feet but you'll pay a lot more for them and they can be harder to find (since they aren't usually needed by most consumers). The only thing I can say is buy quality cables from a good "certified" HDMI manufacturer (there's only a few that are certified: Monster, Sony, JAE, Audio Technica and a few others). IF you have to pay retail price for a 20-25' cable, expect to pay probably up to $200 for the best, and around $125 for the mid-line.
Be sure you know the difference between DVI-A (analog) and DVI-D (digital), and check which one is on your video card before buying a cable to match. Buying the wrong one (the pin-outs look almost identical) will result in no output.
Remember, in the end it all depends on the original source material as to how good it looks on the tv. If you have quality connections between all source and output, it'll make anything look as good as its supposed to. Its not going to make a 30 minute 50mb AVI look stellar! I assume you're downloading Hi-Def digital content (probably from a 1080i source such as hi-def tv episode captures).