The Gigabyte GA EP45-UD3L. I think you have to use the Custom Builder option to have it available.
You're right, Drac - I tried clicking on Custom builder and I found it. But since I'm a wee bit leery of putting together a PC using custom builder, I stuck with the Value Series.
Ok, here we go - I've selected (Value Series, L panel, "Customize Button"):
CPU - 2.6 GHz Intel Core 2 Quad Q8400;
Heatsink/fan - took default;
Motherboard - Gigabyte GA 73PVM;
Memory - default - 2 GB;
Hard drive - default - 500GB 7200 RPM;
CD / DVD / CDRW - default;
Video card - 1 GB GeForce GTS 250;
Operating System - Win XP Home;
Case - Apevia Silver X Dreamer;
Power Supply - 500 Watt Antec Earthwatts EA500;
Networking - default;
Wireless networking - Wireless B/G PCI Adapter;
Sound card - default;
Speakers - none - will use these from current Dell Dimension 8200;
Floppy drive - 1.44 Floppy Drive (I never know when I might order a DOS game from Amazon.com/Ebay that comes on a 3.5" floppy disks);
Keyboard/mouse/card reader/accessories 1 & 2 - none;
Will use the keyboard, mouse, and CRT monitor from the current Dell Dimension.
Warranty - default;
price is now $827.