Jason, you can use Nik's software on jpeg files, though I have only tried it with Silver Efex and Color Efex, I'm assuming that the others will work, as well. Naturally, one won't get as muc leeway with a jpeg file. I'd suggest reading about their software in detail here: Nik Software, Inc. | Welcome
All I can tell you is based on my personal experience. Don hounded me for months to get the software. Finally, they had a discount offer on the Complete Collection and I went for it. Never regretted it - only regretted waiting for so long!
The way I look at software is that if I can spend money on a camera and lens/lenses...and not spend the money to get the best quality image output then I am wasting my funds on the camera in question.
I don't want this thread to get off into the pluses of different softwares from Aperture and Lightroom and Photoshop, etc., because we've got the Image Processing forum for those very worth while discussions - however, without good software I do not believe anyone is ever going to be able to achieve their own best.
Jason, just to quickly answer your questions. These programmes don't work on RAW but you can export the RAW files as TIFFs. NIK DFine is good but I'm not convinced it is better than other NR packages. The only real advantage is the localised application of NR through the U-point technology.
All NIK products I've used are excellent, but they aren't cheap.
Right, I guess I was thinking that what I do is turn my RAW files into DNG while importing to Lightroom, then Nik turns them into Tiffs...which I then export as jpegs or upload via Lightroom to Flicker, where again they turn into jpeg files. I did, however, do some editing on some of my daughter's jpegs recently...and have improved some old jpegs of my own via the Nik software. I rarely have used Define. I have yet to try Viveza, though I know I should.... I am very seat of the pants and intuitive in my approach, but I do enjoy the products very much and have found that for me it was worth the expense.