Whether you expand by force or by settlers, there's a limit to how many cities you can have without sending your economy into the toilet. That's a fundamental difference between Civ4 and all of its predecessors. In all previous Civ games, there was something called "corruption" that caused some of the commerce from your cities to be lost, depending on how many cities you had and how far they were from the capital. But a city didn't inherently cost you anything, although its buildings did. So you could build as many cities as you wanted as long as you built no buildings in them.
In Civ4, buildings are free, but cities aren't. There's no corruption or waste any more, but each city costs gold from your central treasury to support, and again it's dependent on how many cities you have and how far they are from your capital (or, later on, your Forbidden City and/or Versailles). As cities get bigger and cottages mature, more commerce flows into the coffers, so you can support more cities; also, with appropriate tech you can build Courthouses (cuts city cost in half) and gold-amplifying and commerce-amplifying buildings (Market, Harbor, Grocery, Bank, Airport). But none of this is available at the start of the game.
An early rush against a close neighbor is one way to play the early game, and if you have a REALLY close neighbor it's sometimes mandatory. But remember, this is a REPLACEMENT for settling with settlers, not an ADDITION to it. So either you build just one city (plus your capital) to crank out the troops, and take the enemy's cities INSTEAD OF building your own, or else you attack a bit later but you raze almost all the enemy's cities instead of keeping them. (Bear in mind, though, this latter strategy risks having some other AI come in and settle before you can afford to.)
I generally either do or don't do a rush right in the beginning (usually not). If I do, it's done with axemen. I'll add swordsmen if and only if I have iron in my cultural borders when I hit Iron Working, which usually happens during the war. I won't build any more cities to get the iron, and I won't wait until Iron Working to do it. Whether violently or peacefully, my goal is to get enough cities going to maintain tech parity and prepare for my main wars down the road. I'm trying to master Monarch so I'm still playing with what number is optimal. I know that 7 is too few, and 12 too many, from sad experience. Somewhere between those is optimum, then.
My later wars come in rhythms. In my most recent game, playing the Vikings, I didn't do an early rush, settled 9 cities, and went to war in the Middle Ages using Berserkers and cats. I clobbered the enemy (Mayans) and reduced them to a vassal, but my economy took a terrible hit from war weariness; I had my slider down to 10% research and I was losing population in my biggest cities, including my capital and my main research city. This put me behind in the tech race a bit. My next war, against Willem, I fought when I had upgraded my Berserks to Rifles and added many more rifles, and took them and Cannon against Willem only to discover that he had already researched Assembly Line (although he had very few Infantry). I still managed to pull that off by researching AL as quick as possible myself and upgrading my Rifles to Infantry.
I'm now third in tech, behind the Ethiopians and Mansa Musa, and we'll see whether I can crush Ethiopia (the tech leader) with my tanks, artillery, and marines, and then go after Mansa, before the latter (on the other continent) launches his spaceship. No one else is seriously in the running, and if that doesn't give me a Domination win I can conquer a bit more or else do a spaceship myself. I may pull this off yet, and if so it will be my first Monarch win, but obviously I'm not optimising somewhere.
It's a difficult balancing act is what I'm saying, conquest versus your economy. You had to do it carefully.
but to no avail. Any tips or help would be appreciated.
at 100% for a few turns when you have no 
will put your economy in a deficit situation. You can play in a deficit for a short time, but you will eventally have to balance your cash or all of your units will disband.
