Uhmm...i'm out of that "market" since quite a while,. as the pc i have is ok for me, but let's see if i can give some tips at least.
First of all, have a look at Dell and HP website. They both make some really good and not so expensive computers with everything you need in them.
1. Tower unit with the ability to play DVD's.
This shouldn't be a problem, any computer that you buy today should be able to play DVDs without much troubles.
2. Something fast enough so that I'm not watching MPGs and WMVs like they are flip books.
Same as above, just be sure to don't get less than 512MB of RAM (1GB would be best)
3. A larger monitor, perhaps flat screen?
I have 19" flat LCD screen from Sony. I can't say anything else but GO FOR IT. Amazing quality, big screen, clear view and, more important, not so much space on your desk.
4. Software to capture videos and edit them, also to add music/dialogue.
To capture videos you also need an hardware piece. Pinnacle has the best solutions for this. (Pinnacle Studio can be bought among with a video capture card at a not so expensive price)
5. Software to create and animate figures.
I'm not an expert in this one... can't really say anything!
6. The best firewall/virus protection.
Virus protection -> AVG Free version. It's a free antivirus and does the job in a good way. (
http://free.grisoft.com/ )
For firewall...err... Zone Alarm, i think. But, again, i'm not an expert as i just use the standard XP one...i think you need a special one only if someone is really trying to target your pc. For normal, random attacks, the XP one does the job for me.
7. Speakers.
You could do the same i did: they are included in the LCD Monitor. An all in one who, again, saves space on your desk and does the job. (providing you don't want the super high quality sound that just some dedicated hardware provides...but i'm not an expert in this (again!))
8. Ergonomic mouse.
Err... go to a shop and ask to try them! Logitech and Microsoft usually sells good things.
9. Software to make phone calls on the PC.
I use VoIpBuster, but i know that in USA there are a lot of alternatives. The most famous is Skype which is good. In any case, anyone is good if you use one of those headphones that include a microphone with noise reduction (you can buy one from Creative at about 20$!)
That's all...i hope some of this tips will come to help