I am in the planning stages of my Hackintosh atm.
I was looking over the specs I have and I realised I was building a Mac mini!

Here are my specs:

1.6ghz C2D
1gb DDR2 RAM
80gb IDE HDD (old)
Motherboard with GMA 950
SATA "Super drive" (DVD-RW)
Belkin wireless PCI card (old)

This takes me to £150 - much less than a Mac Mini.
Once I get it up and going, I will do a xbench and see how they compare.

In case you are wondering, I am not planning on keeping the GMA950 for long - I am waiting for the DX10 graphics cards to come down in price

i'm not sure if they're made with P4 technology, but if a miniITX board supported SSE2/3 then you could use that, throw it all into a mini ITX case, get an apple sticker, slap that on top and have a powerful yet small Hack Mini.

i call my computer a Hack Pro as i'm running everything on here....soon i'll be able to get Final Cut Pro HD too, more specifically i'm looking for soundtrack pro.

when i get home i'll even have pictures/videos of the mighty mouse i fixed today. damn they're biznitchy. too bad two pieces got thrown away or i would have put it completely back together, but it works, and thats what matters.

You could try running on the pentium dual for less the price, but the c2d will suit your needs just fine.