By external reset switch, you mean the one that is mounted on the rear I/O panel? Those are pretty chancey. I know Gigabyte and Asus have them on high end motherboards, but you should check out product pictures or specifications before buying, if you need it. There's no guarantee that its a feature, even on some high end MOBOs.

HP Media Center machine
MOBO: ASUS OEM board
CPU: 2.6 GHz Socket AM2 Athlon X2 5000+
RAM: 2 GB Qimonda PC2-4200 DDR2 RAM
GFX: Sapphire Radeon HD 4850
HDD: Seagate Barracuda 320 GB SATA 7200 RPM