While I build my own (see my profile), get Maximum PC magazine. They have a lot of advertisements for custom built computers. You decide on what you want. $3k is a little overkill right now. You can get a very nice quad core with plenty of ram, fast video card and big hard drive for less than 1/2 that price. AMD will be a "cheaper" system build. Intel i7's will cost you the most (DDR3 ram is expensive along with the chip and motherboard, but you will have the fastest on the block). Right now, AMD has come out with the Phenom II CPU and Dragon build system (all AMD... chipset on motherboard, video card and cpu).
There are a lot of options out there.