I recently (as in over the weekend) upgraded my HTPC which is the only PC in the house that was still running on a single core (Pentium M) CPU.
The upgrade was to a fairly modest, but well appointed micro atx mobo: The Asus P5B-VM, which sports the Intel G965 chipset. I mainly chose it because it supports Core 2 and quad core (in this case a e6700); has an eSata port; 2 PCI + 1 PCI-E (x4) + a full bandwidth PCI-E (x16).
I'm somewhat ambitious with my usage goals for this system. Freeview HD TV (incl streaming to other PC's in the house); Gaming + Media/file Storage for network sharing.
The issue with gaming of course, is that you need a hefty graphics card (I installed a 8800GTS 512 G92) and that these cards are not really suited to a HTPC because of their high power consumption (mostly I'm concerned about idle power usage as this is how it will be spending the majority of it's time)
However Asus has an excellent solution that has suited my purposes brilliantly. AI nap (a utility that came with the mobo) simply runs in the system tray, and if you select it (or hit the power button) it will turn off the video card; sound cards; slow all the system fans down; but still allow the cpu and HDD's to function normally, which means the system will still perform as a media server for other remote PC's while reducing power consumption over a fully powered system.
Anyway, thought I'd mention it because I think it is a great option for someone who wants to run a 24/7 system with a high performance videocard installed. I'm not sure if other mobo manufacturers have something equivalent?