After many hours working I've noticed thoes lines too, but not only on leopard also on windows. Solved it temporarely unpluging and replugging from the video card. (let it rest a while)

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CPU: AMD Athlon 64 3000+ @2.3Ghz - Motherboard: ASUS A8N-SLI nForce4 SLI - nForceLAN by eno - SuperNForceATA by Medevil - RAM:2 GB DDR 333Mhz Audio: ALC850 Video: XFX nVidia GeForce 8400GS 256Mb (0x06e4) QE & CI from Chameleon 2 RC4Ethernet: Realtek RTL 8139 Series (working out of the box) OS: Snow Leopard 10.6.4 (aryajuanda's guide) + Windows XP 64bits