AMD = slower and cheaper.
Go Dell or if you want the best cost/performance BUILD IT YOURSELF!
cyberpowerpc.com = good prices and good computers.
dell.com
Also, the components you wanna look for are:
CPU/Processor: get a comp with an Intel core 2 cpu... popular ones include the e6600(dual-core), e8400(dual-core), and the q6600 (quad-core)
Graphics card/video card: Nvidia Geforce cards are great. ATI is doing good lately too. NVIDIA: Go for any of the 8800 cards, 9600 GT, any of the 9800 cards, GTX 260... or if you can go crazy with the spending, a GTX 280. ATI: 4870 or 4850... if you got lotsa money grab a 4870X2
RAM/Memory: 2gb MINIMUM. any speed will do but get DDR2 or DDR3 memory.
OS: Windows Vista. Despite what noobs say... it's better than XP. Go for 64-bit version if you can, it's more future-proof.
The manufacturer will give u a decent motherboard, PSU, hardrive for your rig so im not going to go into detail about those.
Hope this helped.