Considering getting something from the Dawn of War sale, not sure what to get. I will most likely be into a good single player campaign, not so much multiplayer but I do like multi sometimes. I like lots of bells and whistels these days since I finally have a decent computer after decades of being 10 years behind everyone else.
If I was to buy some DoW things, what ones would be considered essential?
Speaking from personal experience, DoW II has a nice single-player component, and with the graphics turned up it definitely meets the requirement of "bells and whistles".
Of the three games (of course, you could just buy all 3 for $30...):
The base DoWII has the longest campaign, and a surprising amount of freedom. You can only play as the Space Marines, though, which slightly limits your options.
Chaos Rising has a more linear campaign, although the morality system gives at least a little replay value. The missions also tend to have more depth and variety, whereas DoWII insisted on throwing 'defend the location!' missions at you every few days. Still restricted to Space Marines, though.
Retribution does away with GFWL, which was a nice bonus, and the campaign lets you play as any of the 6 in-game races. The downside is, the campaign is basically the same for each race (for the most part, they slightly reorder the missions, and there are tiny changes to some objectives), so replay value dies off pretty quickly. If you're looking to do multiplayer, make sure you get this one, since AFAIK it's the only one that's still active.
Plus, Retribution's having a free weekend, so you can give it a try beforehand!