Hello,
Right now I've got a Asus P4P800 w/ a P4ht 3Ghz@3.6Ghz but I am thinking to change the CPU, and I wanted to know if some one knew if I can install a P4D or even better a C2D E4300. It has the FSB at 800Mhz like my P4ht and the same socket...

Since it's socket 478, the fastest option is probably an overclocked dothan Pentium-M CPU on an Asus CT-479 adapter. The problem is that dothans don't have SSE3. I don't know what the performancew penalty of SSE3 emulation is in OS X.

Better stick to your 3Ghz CPU I say, if it's a prescott one it will support SSE3. My 3.4Ghz CPU is one running happily on my P4P800Deluxe mobo. Nothing really faster out there, besides upping the Mhz'es just a little won't make that big a difference. There is quite a big performance difference between a P4 SSE2 and a P4 SSE3 capable CPU though.
My P4P800 deluxe motherboard supports the new prescott core, but I am not sure your P4P800 motherboard will support it.

How can check what is my CPU?
Is the prescott core the last core? and is it really so fast? Apparently a few people on internet used the prescott on a P4P800x.

I found that but they don't tell anything about the core...
http://www.cpu-upgrade.com/mb-ASUS/P4P800-X.html

I don't think the diff on windows is that big.. but with OS X it probably is.

Because people have to translate SSE3 to SSE2, and with the prescott that isn't needed anymore.

Makes me want to get a Prescott LOL. *goes to Ebay*

As you explained to have a Pentium 4 HT 3,0GHz Socket 478, you're CPU probably could be a Prescott processor!
You can simple check this with the free tool CPU-Z. Or by knowing your Processors 2nd Level Cache size.

The Northwood P4 3GHz only had 512kB 2nd Lvl Cache, as for the Precott P4 3GHz was equipped with 1024kB 2nd Lvl Cache.
Due to the fact that Intel only released this two versions of 3,0GHz HT P4 for socket 478, the possibility stands 2:1 for Prescott.

And yes: For Socket 478 the Prescott-Core was the last released architecture.

Well apparently I have the Prescott..
Thanks