Below are the minimum and recommended requirements to run PCSX2

Minimum Requirements:
Windows 2000
A CPU that supports SSE2 (Intel Pentium 4 or AMD Athlon 64)
A GPU that supports Pixel Shader 2.0 (NVIDIA GeForce FX 5200 or ATi Radeon 9500)
512 MB of system RAM

Recommended Requirements:
Windows Vista
Intel Core 2 Duo or AMD Athlon X2
NVIDIA GeForce 8400 GS or ATi Radeon HD 2400 Pro
1 GB of RAM

Notes:
- To use GSdx's Direct3D 10/11 hardware renderer, you will need a video card and operating system that supports DirectX 10 (Windows Vista and NVIDIA GeForce 8400 GS or ATi Radeon 2400 Pro).
- To use GSdx-SSSE3, you will need a CPU that supports SSSE3 (Intel Core 2 Duo or AMD FX). To use GSdx-SSE4.1, you will need a CPU that supports SSE4 (Intel Core 2 45 nm CPU or AMD FX).
- Keep in mind that even if your PC is new and/or it runs Crysis 2 fine, it doesn't mean it will run all emulated PS2 games at full speed.
- PCSX2 usually uses 2 threads (cores) at most*. That means that if you have a 4-cores CPU, the maximum CPU utilization you'll see is 50%, and usually a bit less. If you have a single-core CPU, most games will run slow.
- Newer nightly builds can utilize 3 cores when using the new MTVU speed hack.
- Laptops: Make sure power settings are set to "High performance". If you have dual GPU, make sure the external one is selected.