The Core 2 Duo E6300 is 1.87GHz and it belongs to the first generation of Core 2 processors, any Core 2 with higher frequency, and even the Pentium Dual-Core E5000/E6000 series should be able to handle PSO2.
The problem for anyone using a Mac could be the graphics card, since usually Macs have low-end to mid-end graphics card. The first iMac that had a graphics card as good or better than the 7800GT was in the early 2008 iMac series, with the optional Geforce 8800GS. Previous iMacs used cards like the Radeon HD2400 or HD2600, which are quite a bit below the 7800GT...