Rise of Nations and Homeworld Series! A must... you will enjoy them.

As far as recent titles go, Sins of a Solar Empire wins hands down.  Really a solid, streamlined title that I'd recommend to any RTS fans.  For classics, my all-time favourite remains Warcraft III.  Absolutely a phenomenal title.

I liked Supreme Commander, but personally I found you spent way too much of your time juggling build orders and micromanaging workers.  As far as command and conquer goes... the series ended at RA2 and I refuse to recognize the trash they've been churning out since.  I haven't played DoW so I can't speak on that.

Be careful with Empire total war, I still haven't gotten it to work well on my PC, worst $50 I ever spent (and I absolutely loved Rome total war too). I wish I had spent all of the hours of fustration playing Sins of a Solar Empire even more...

Also I actually liked the Original LOTR battle for middle earth more than the sequal, but they are both pretty good. Rise of Nations is a must play for RTS fans. C&C Generals is still my favorite from that series for some reason. I'm probably just echoing other people but you get the idea.

I have a Q6600, GTX 285 (Before that a GTX8800) and 3.2 gigs of ram on XP Pro, works fine. 

when u play homeworld2 there are some things to remember to get the most out of it./

H: sends your resource collectors to mine the closest asteroids

f: focus on a particular ship so you can zoom right in

ctrl F: select a fighter thats part of a squadron and zoom right in on it. nice to go in with attack waves. just like watching star wars as they duck and weave among the big ships.

Target subsystems on big ships. u can disable indivdual turrets, engines, construction modulkes etc

Do yourself a favour and install the SVN mod to upgrade the dx9 effectgs before playing te campaign.

edit the config file to 1680x1050 and above and play with the grapics settings. its worth it. And the game really does look 20 times better than sins still.

if u build a hyperspace module on the mothership, carrier or anything else, ships near it can hyperspace. But they cannot get back. if the hyperspace vessel jumps with your fleet, obviously it can jump back if u have enough resources.

Make sure u dock fighters before the jump or they will get left behind

battlecruisers with hyperspace modules are nigh unstoppable with the proper fleet support.

modidng community still very strong, and hw3 is rumoured to be in dev.... well as much as mechwarrior 2009 is

In mount and blade you can sort of command your troops...especially with some mods.

I've got the original World in Conflict and I think I mentioned it earlier.  I didn't pick up the expansion cause I thought I read somewhere that it was all the original missions just reversed so you were playing the Soviet side.  Plus I live in the Pacific NW and it's a little disconcerting assaulting somewhere that you lived.  But agreed the game does look phenomenal.

Also, I don't have a link but somewhere on gamespot I saw some screenshots of a game called Order of War that looked impressive.  Although it's another WWII era strategy it looks pretty polished.  My only problem with what I read is some of the commands sound like they may have been simplified compared to other similar titles.  In one of the previews I read it sounded like the scale of the battles may even be on par with SupCom.  Whether that will in fact be the case remains to be seen. 

At the very least it might give us some inkling as to what square enix can do with an rts.  I'm afraid they're gonna change what for me was a winning formula with something that looks great but without the substance.  I remain concerned. 

That's funny...the reverse is true for me. I grew up in Ohio, and I enjoy destroying the farmland maps more than any other ones....There's just something about dropping tactical nuclear weapons on red barns, tractors, and grain elevators that is deeply satisfying to me.

Earth 2160 - Is good usually overlooked 4 playable different tactics to playing them all, not just carbon copys. It was decent I still have it installed.

All these postings are great suggestions. My favorite RTS of all time was Empire Earth 1 and 2. It was a great sandbox RTS with a massive tech tree. I am not sure how active the mutilplayer is after they shut down the original servers but the single player campaigns were pretty good. I remember one where you had to secretly build an army while the warlords in the area kept extrorting money. They would patrol your area and if they spotted any military units they declared war and wiped you out.

Keep away from Empire Earth III. It destroyed the franchise.

Check out the open sourced Spring Engine RTS, its free and has several different mods.

I'd avoid Company of Heroes at the moment.  Relic royally screwed up on the latest expansion; balance is FUBAR and the game is really stale at all levels of play due to certain units/strategies dominating.  If you do get it, don't get Tales of Valor, get the Gold Pack instead.  While standalone and offering full multiplayer access, Tales of Valor doesn't have a real campaign.  The original and first expansion did, and are actually pretty well done as campaigns go.

Rise of Nations is a classic.

Dawn of War: Dark Crusade was vastly better than Suckstorm, and has some strong mod support.  Despite the two new factions, I'd actually uninstall Soulstorm and play this over it.  Look for the DC Pro Mod, which adds some spiffy new features and units, much better AI, and dramatically better balance.

Command and Conquer Generals: Can be had cheap with the First Decade pack, and is good mindless fun.  Balance is nonexistant in the expansion, but that doesn't mean it can't be fun single player.  The GLA missions were fun, and the Generals Challenge was good for one or two playthroughs.