If you want to buy a brand name system, I'd recommend a Invalid Link Removed or Invalid Link Removed. Dell has some used and refurbiushed systems that are good at a low price. The plus about Dell is they have really good service and support. Alienware systems are mostly for gaming but still look cool and have lots of options.
If you want to build your own, its easy. I'd look for a motherboard deal on Invalid Link Removed and buy a good AMD chip that has the memory, fan and everything else with it. I usually buy most other stuff at Invalid Link Removed but they have pretty poor service so make sure you know what you're getting. Otherwise, a local shop should have what you need. If you're going to build your own make sure you get some static wrist bands and be very careful when installing the chip.
If you need any help, let me know. I'm an A+ certified hardware tech (wooo!) and I worked at CompUSA for two years doing support (don't buy from CompUSA).
Good luck!