At minimum you probably want some kind of graphic/photo enhancement application if you include stills. Photoshop may be overkill for a lot of people unless you also use it for other purposes. There are many similar cheaper applications.
Borris Red and/or After Effects open up the possibility of more advanced special effects. Both are expensive. Again what are you going to be doing?
Download a copy of VirtualDub (free) is useful. It takes care of a lot of pre processing like removing video noise, that kind of thing. Also download Satish's collection of plug-in also free but worth their weight in gold. There are few other third party application that work with Vegas as plug-ins. Other stand alone applications. Many that work with uncompressed AVI that Vegas can output to or you can frame serve again with one of Satish's plug-ins.
If you're just starting out, learn Vegas first. You'll be surprised how much it can do all by itself.
Vegas IS a soundmixer. But if you want to work with audio to the very finest detail, you'll appreciate Sound Forge. It is a standalone, but works seamlessly with Vegas like a plug-in. The Screenblast version is simpler and cheaper - you might be happy with that?
Also Sony Noise Reduction rescues a lot of audio from background noise, camera noise etc. It'll process your vinyl records too, when you get around to burning them to CDs.
Accoustic Mirror is included with the dearest Sound Forge. It will take a recording of an accoustic environment (like a church or a warehouse) and wrap it around a studio recording (dialogue etc) until it sounds like the dialogue was recorded in that church or that warehouse! How do you like that?
I don't think Excalibur will let you do anything Vegas can not already do, but it simplifies and automates some tasks. A lot of people seem to like it.
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Sound Forge, Photoshop Light (I think it might be called "Impressions") and DVD-A for simple DVD authoring or Adobe Encore if you need a fully functional DVD authoring tool.
Maybe Executive Software's Diskeeper Professional for defraggging. Stay away from Norton System Works though!
I have added two boris products - Graffiti 3 for titling and FX for special effects. Have not really used FX much yet except for transitions but Graffiti is great for end of video credits. More flexible that the vegas generator and easier to import formatted word documents.
I use Photoshop Elements on every single project. Satish's new Wax plug-in may eliminate the need for it, but I haven't had the chance to check it out yet. Elements 2.0 is very inexpensive and has most of the retouching features of its big brother. I know some people use GIMP (a free application available on the web), Cinepaint (another one somewhat based on GIMP), and JASC Paint Shop Pro (more features than Elements for about the same price). If you don't have any of these yet, Download Wax 2.0 from www.debugmode.com and see if it offers what you need. Don't forget to download Winmorph from the same website. Awesome Program!
for particle FX, ParticleIllusion is GREAT!! Also, as a standalone app, Dynapel's STEADYHAND is a great shake stabilizer tool. I'd also recommend a series of add-on codecs like HUFFYUV, 3iVX, and the Matrox DVC50.
Over here in Mossland (Oregon) I find myself using the following all the time:
(1) Vegas (duh)
(2) Paint Shop Pro (very powerful, yet cheap)
(3) SteadyHand
(4) Studio7 (Used strictly for the excellent TitleDeko titling program inside it)
(5) Nero (for burning anything to anything)
(6) Nero's CoverDesigner (for labels, DVD jackets)
and because I got lucky on eBay last year:
(7) CinemaCraft MPEG2 encoder
(8) DVD Maestro (DVD authoring)
I'm sure Adobe makes fine products, but for some reason I get the shakes when placed near an Adobe interface. Must have been traumatized as an infant, or something, I guess.
Next project is to lean about Acid loops. Played a lot of piano as a kid, though I'm not sure there's any relevance there.