Dual-core processors, particularly AMD Athlon X2 or Intel Pentium Dual Core models, are the norm in value PCs, though single-core processors (AMD Athlon 64, Intel Celeron) can show up in really cheap models. Most Pentium Dual Core processors are based on the same architecture as the Intel Core 2 Duo processors, with a little less L2 cache, clock speed, or FSB speed. For example, the Pentium Dual Core E5000 series are similar to the Core 2 Duo E7000 series processors. I recommend a dual core AMD or Intel processor because it's a must for today's attention-challenged, multitasking PC users.
Look for at least 2GB of RAM: The more memory, the better. More memory allows you to do two things: open up more programs and windows at once, and it makes multimedia processes (like editing photos) faster. Have you ever opened up Windows Media Player, IE, Photoshop Essentials, and e-mail all at the same time, then seen your system slow down and your hard drive activity light continuously blinking like a flirting Cyclops? That's the effect of having too little RAM.