Congratulations :)
I recently got my dream machine custom built. 

My computer specs;
M/B ASUS S775 X38 DDR3 P5E3 Deluxe ATX
CPU, INTEL S775 C2Q QX6850 3.0G 1333 RTL
MEM OCZ DDR2-667 4GT KIT XTC OCZ2VU6674GK
PNY VCQFX3450-PCIE-PB QUADRO FX3450 PCIE
MS WINDOWS XP PRO 64 BIT W/CD & MED SP2C DSP
SNDCRD, CREATIVE 70SB079000000 X-FI AUDIO
LITEON 20X DBL LYR DVD +/-RW W/LS RTL
PS, TT TOUGH MODULAR 850W SLI W0131RU
HDD WD 1TB 7200 RPM 16M SATA2 WD10EACS ( 2 of these with RAID mirror done)
CASE TT NO PS ARMOR W/WIN BK VA8000BWS

I would definately reccomend the quad core, getting more RAM, and going with XP 64 bit. It makes a huge difference. With your son building it, you're going to save a lot of money on labor costs, so putting some of it into parts upgrades would be a really great investment.

You are going to have so much fun with your monster once it's done :)