E8500 will give you far better performance and lower power consumption than any P4-based CPU. It will also easily outperform the Q8400 due to higher clockspeed and more L2 cache, all at a lower power consumption. And the extra cores of the Q8400 won't be of any use on anything you want to run on Windows XP. And just in case you want to overclock, the E8500 should hit about 4 GHz on stock voltage (so without drastically affecting power consumption).

Thanks for the feedback! I will definitely be using the E8500 in my next build.

Yep, Core 2 Quad is better for the last hurrah of LGA775, even some modern games (though Apex Legends can't run due to instruction set limitations, a line in the sand that is likely to get deeper.

Core 2 Duo runs rings around the P4 while using less power, much better IPC so it doesn't need the higher clock speed to beat the P4.

Of course, with some chipsets, they cannot go any higher than P4, like my sad old 915 chipset system

There were some XP-ish games that had pretty good multi-core support. A lot of Xbox 360/PS3 ports were written that way. Prince of Persia 2008 is a good example, I switched from a C2D to a Q9300 (2.4GHz, 6MB) and saw some improvements.

That said, an E8500 is also a good choice. Really neither of those would be "wrong."

Matth79 wrote:

Of course, with some chipsets, they cannot go any higher than P4, like my sad old 915 chipset system

I noticed this about half of the possible boards that I found for this project. Many of them only support the Pentium 4/Celeron lineups. This would have been a step down from what I am currently using for Windows XP gaming.

I was using a pentium-4 dual core and WinXP was very sluggish.
I Up graded to a Core-2-quad Q9300 and performance greatly improved.
I previously had XP running on an HP z800 with dual quad core Xeon for a total of 8 cores and 16 threads.
Even though the games will not support more than 2 cores XP can benefit from the cpu load with a quad core cpu.