The Age of Empires games have already been mentioned, but if you enjoy them then you may also want to look at Rise of Nations.

my personal favorites games of all time were

rise of nations and age of mythology for actual online playing

stronghold for roleplaying and just being able to build an awsome city.

Age of Empires 2 (do NOT play 3, the 10 year old AoE2 is much much better)

Rise of Nations

Europa Universalis 3

Sins of a Solar Empire

Total War games are good but they're not RTS

Sins of a Solar Empire is a fine RTS - its also an empire building game at the same time. Take a look:

maybe wait for AOE online which should come out later in the year. 

However, you really need to mention whether you will be playing online or not...most RTS games (or the ones I've played) are a completely different beast when playing online versus just doing the single player campaign. Also, some RTS games simply do not have an active online community so if you are going play online then I'd choose carefully. 

wasn't that turn based as civ?

I forget :p

No, RoN is an RTS...and a very good one if I might add

Not quite. It was a weird mix of Civilisation and Age of Empires. It was real-time and had Ages/Epochs, but had territories based on your city position and instead of destroying town centers they simply merged with your territory. It bizzarely worked.

it had a civ tech tree then, no? well there was influance (civ is fun too though turn based.... takes more time ^^)

Age of Mythology

Empire Earth

Red Alert 1 and 2 

RTS/TBS games which I like even more than RTS games:

Rise of Nations

Rome Total War

Medieval 2 Total War

I personally didn't get into Age of Empire 2 as it's really old. Though Red Alert 1 is even older.