|The AMD Phenom quad not only has L2 cache per core, but also has a shared L3 cache. The Intel Core 2 Quad is better/faster, but it's also more expensive. The new Phenom II is giving the Intel quad a run for it's money though.|
The Phenom is a high wattage CPU, requiring almost twice as much as the Athlon X2 dual-core, so make sure you have a decent power supply. You should also have DDR2-1066 rather than DDR2-800. DDR2-800 will work, but it will bottleneck the system somewhat.
If you go for an Intel system, choose a board based on an Intel chipset such as the P45 rather than an nVidia chipset, but do not select a board made by Intel.