Are your graphics turned down a lot? I have a computer that runs WoW quite laggy at the "normal" settings even without addons, but with my video settings turned down low, I can easily run Vuhdo, Skada and DBM and it's only about as laggy as it was at the normal settings.

It's hard to compare machines, though.

I use a considerable number of Add-ons, though always striving to find the "leanest" ones, but if your computer starts lagging with just DBM and Healbot, something else is wrong.
DBM is module based by default, which means it would not load data for BWD when you are inside Baradin Hold. Healbot is a luxury that can be avoided by simple mouse-over macros. If you are using it just for raidframes, get something leaner like OUF_Freebgrid etc.
But that said, i think you need to look into your video settings.

I keep all of my video settings to the bare minimum, no doubt about that. I don't blame the game, I know it is my computer dying, wasn't meant to be a complaint at all. I really was just curious about how good the specs on a machine would have to be to run WoW and all those add ons smoothly. After all, I know my computer is dying, and I will have to replace it.

I am currently waiting for Intel to fix the problems with the Sandy Bridge line, but until then I am stuck with my 2003 Dell, single core running 30 fps at the best of times, sometimes less than 10 fps in a 25 man raid. I don't do those very often, obviously, because I usually have to bail due to not being able to see anything.

But what I have found, based on experiments conducted with the assistance of add-on control panel (if you don't have it, get it!), is that add-ons are not the make-it or break-it part of running a smooth game. I use 22 add-ons, including Grid, IceHUD, DBM, and Auctionator. I use AOCP to quickly turn off those that are not immediately needed, so I actually only use about 18 at once, and I get about 30 fps, as I mentioned.

But you know what, under similar conditions, with all 22 running, I only dropped to about 25 fps. So I turned everything off, and only came up to about 37. Those 7 fps are not worth it to lose all the functionality I get from the add-ons.

Take hints from here and the add-on spotlight column to find the smaller footprint add-ons, but don't avoid them just because you are afraid they are slowing down your performance too much. It really does not make a huge difference. :)

That actually happens to me from time to time. I keep a can of compressed air on my desk and, if need be, open the case and have at it with the air. This is a particular problem for me because I have a videocard with a heatsink but no fan attached. I refuse to buy a new card at this point because I'm waiting to upgrade the whole machine. (Most of the components are probably 3-4 years old and it was pieced together on the cheap at that point.)

TL;DR: If you have more slowdowns than you can account for - or than you used to - make sure the inside of your case is as dust-free as possible.