Didou said:
A dual-core is much more suited for audio/video. You can start off with an Athlon64 X2 3800+ or a Pentium-D 820.
I agree with didou on that one, but I don't believe that this kind of system is attainable under your $1,100 budget and if all you're gonna do is play music, videos, and the sims, a $500 Athlon XP/Sempron or P4 system will easily suit your needs
...here are some ways to afford what you want to build.
1. buy everything you can on EBAY, you will save an incredible amount of money as opposed to buying from stores (both online and brick/mortar), buy from high feedback sellers and you won't get burned.
2. buy OEM whenever possible (unless the price difference is minimal, why pay all that extra for a box and a paper manual you probably won't even need?), most of the components you will use will (a)use firmware, and thus not require a driver at all, (b)your operating system will install a suitable driver for you, or (c)if you need the manufacturer's driver, they are free and easy to get online. The only thing I would recommend you buy in full retail packaging is the motherboard.
3. since you don't know what is inside of a computer case (given the question you posted), I will assume that you have no intention of tweaking (overclocking) your system, therefore, you can get away with cheaper motherboards and RAM chips as long as they meet the speed requirements of your CPU
4. since this is your 1rst system build, Windows XP SP2 is your best bet for an OS that will work the 1rst time you turn on your computer
-hope this helps..