It actually use exact same motherboard , MCP73T-AD, as Ax1800 and Veriton VX270 series. NVIDIA GeForce 7100 and nForce 630i capable of handling C2Q processor, such as AX1800. ( if you decide to do processor upgrade )
As for the video card, depending on your budget or your definition of "game rig", you might not even need to change the case. There are Low-profile video card such as "SPARKLE SX96GT512D3L-NM GeForce 9600 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready Low Profile Ready Video Card".
Recently, I updated my AX1700 with AX1800 BIOS P01.A4 Date 11/19/2009. ( upgrade at your own risk ) and my stock PI CPU rating jump to 5.7 under Windows 7 64bit.
p.s. if you flash your mobo to AX1800, expect to re-active your Windows 7 again, as it will detect as "another pc".