http://www.intel.com/design/motherbd/sv/sv_proc.htm
The motherboard's list of supported CPU's ^^
http://processorfinder.intel.com/Details.aspx?sSpec=SL6PG
3.06GHZ P4 533 FSB ^^
So to answer your questions
Yes I know, no it doesn't support anything higher, yes the 3.06 has HT, and yes your chipset has support for it (
http://www.intel.com/products/chipsets/845PE/index.htm)
I can't seem to find the 3.06 for sale anywhere, and as far as upgrading, I'd say avoid getting another mPGA-478 chip, either save up for a new set-up, or snipe at good deals on AMD processors for Socket 754 or 939 (939 is gonna give you some more options)
Another idea would be to go after something on LGA775 with a VIA PT880 based motherboard that has both AGP 8x and PCIe (I know ECS and Asus both have a variant on this, and I think ASRock does too)
these motherboards:
http://www.ecs.com.tw/ECSWeb/Products/ProductsDetail.aspx?DetailID=529&MenuID=93&LanID=9
http://www.asrock.com/product/775Dual-VSTA.htm
http://usa.asus.com/products4.aspx?modelmenu=2&model=1126&l1=3&l2=11&l3=0
http://www.biostar-usa.com/mbdetails.asp?model=pt880+pro-a7c
And if you want some more information about the PT880:
http://www.via.com.tw/en/products/chipsets/p4-series/pt880_ultra/
Most of those boards are DDR/DDR2 cross compatable, AGP 8x/PCIe x16 cross compatable, and support a slew of other technologies, just add an LGA 775 processor and go. (I'm not sure how they are on Conroe support, they have to support the lower 1.2x voltage to run it)
The ASRock says it supports Conroe, Biostar doesn't say, Asus doesn't say, and ECS doesn't say...
I'd suggest the ASRock due to it's claimed Conroe support, and the other features it comes with...
As far as a CPU, I'd consider one of the nwe 65nm Celeron D's (with 512k L2 cache) or a Core 2, Pentium D Presler would also be an ok chip (honestly P4 Cedar Mill is fine, I'd just stay back from Smithfield and Prescott unless you're getting a REALLY good deal)
