Anything with the name 'Core 2 Duo' or Core 2 Quad would be faster than your current Pentium 4.
The 'speed' (ie 2.5ghz, 3.6ghz etc) is not soley responsible for how fast the processor will be. There are other factors too, which lead to a Core 2 Duo being much faster - for instance it is able to do far more work per ghz than a P4. It's also dual core, (or quad for the core 2 quad) whereas your P4 is only single core. I would estimate that the Core 2 Quad at 2.4ghz would on average be at least twice as quick, if not more in the right circumstances!