general rule of thumb is AMD is great for gaming, while intel is better off for multitasking and video/image editing.
be sure to get at least a socket 775 processor. do not go with socket 478 anymore because it will get old real quick. 3GHz is a good starting point. dual core is even better. if you go with single core processor, make sure it has hyperthreading, and that the motherboard has dual channel memory slots in either DDR400 or better yet, DDR2. i wouldn't go below 2GB of RAM. even better is a motherboard with SATA RAID capablities so you can run a RAID 5 (you'll need 3 drives) or 0+1 (you'll need 4 hard drives) configuration. and with SATA hard drives at very reasonable prices, there shouldn't be a reason not to. you'll want to run two displays for extra desktop real estate, so a good 256MB graphics card will do you well. i'd pick a motherboard that has PCI-Express 16 instead of AGP 8x for graphics interface, just because the technology is just newer and upgrades will come easily.
other considerations: at least a 500W ATX 12V power supply that is SATA ready.
right now i have a P4 3GHz HT with 2GB (4 x 512MB dual channel) and a 256MB video card. i have no problems editing 200+ MB files while having C1, Outlook, IE, and iTunes open at the same time.
check newegg.com -- they have the best prices, shipping, and customer service.