The first thing you need to do is find out which CPUs are compatible with the motherboard. Majke sure to check BIOS versions too...
Other than that you will just need to do what you said, take fan/heatsink off, replace CPU then put the fan/heatsoink back on!
The spec's on HP website say it support E6x00 chips, this gives you the option of E6700 or X6800.
The chipset supports 45nm processors, but it's unlikly that the BIOS full supports them, if your happy to try a newer process it's likely to work, it's likly that the system will report the processor as unknown but work fine.