I have a 4500 with a P4 1.8 GHZ processor.

I see Dell is now selling it with up to a 2.8 GHZ processor so I assume it is pretty much just a matter of what processor you plug in. The questions I have are bit different and perhaps someone can clear them up for me. A local computer Guru says I shouldn't even think of upgrading past 2.0GHZ in that the mother board will start getting flaky if I push it to its limits. I tend not to believe that, but still want to run that out to see if anyone else feels the same way. In that the 4500 has the 845E chipset, it seems like it should be OK unless Dell has changed motherboards since mine shipped. Is there a point where motherboard flakiness begins?
My second question involves buying a new CPU, am I better off buying the retail pack or the OEM version CPU. INTEL wasn't of much help, they gave me a bunch Politically Correct stuff about checking with Dell to find out motherboard manufacturer etc. I know the retail pack comes with fan and heatsink, but is it worth the extra bucks rather than unsticking fan and heatsink off my 1.8 and placing them on new CPU.

Additionally, at what point do I have to flash the BIOS to put in higher speed CPU? I'd prefer not to screw with the BIOS unless absolutley necessary so if I can go up to 2.53GHZ for instance without messing with the BIOS I'd prefer that over flashing BIOS to go to 2.8. (I have the kind of luck that would experience a power failure in the middle of the flash process)

Finally how much of a real world performance increase am I going to see in say a 2.53 or 2.8 GHZ CPU on my 4500 over the 1.8. Would it be significant or possibly in the neighborhood of 5 to 10% overall.

Thanks in advance

Peter