8400gs 512mb Weird horizontal line screen artifacts

[Arck]

Hey, first I want to thank everyone here for all the support as I've attempted setting up my machine these past few weeks and have been doing a ridiculous number of searches here and elsewhere, stealing your info without your knowledge =). I'm actually posting this here from said machine, and have the entire thing running smoothly except for one tiny... detail.

My screen is covered in artifacts that seem to be left over horizontal lines from my windows, icons, etc. etc. Nothing from the mouse cursor, just when I move the windows, type, or do anything of that sort. They appear first on startup as just misplaced lines from my desktop, but the second a window moves over them, they change. If I do a click-and-drag select on the desktop, I get the same result from the grey box.

What bothers me is that these only appear in OS X, I'm running XP and Ubuntu on two other, separate hard drives and my Geforce 8400gs 512mb does perfectly fine. This makes me think that it's a driver problem, but... that' where I'm lost. Have cleaned the card and PCI ports anyways just in case. Attached should be (assuming I did it right) a print-screen of my desktop with the artifacts. So first, relevant video info (I hope)

Graphics by NVIDIA:

Chipset Model: Graphics by NVIDIA
Type: Display
Bus: PCIe
PCIe Lane Width: x1
VRAM (Total): 512 MB
Vendor: NVIDIA (0x10de)
Device ID: 0x06e4
Revision ID: 0x00a1
ROM Revision: NVinject 0.2.1
Displays:
AY765C:
Resolution: 1280 x 1024 @ 75 Hz
Depth: 32-Bit Color
Core Image: Software
Main Display: Yes
Mirror: Off
Online: Yes
Quartz Extreme: Not Supported
NEC LCD1860NX:
Resolution: 1280 x 1024 @ 60 Hz
Depth: 32-Bit Color
Core Image: Software
Mirror: Off
Online: Yes
Quartz Extreme: Not Supported

Am running OS X 10.5.8 Leopard, and installed from iatkos v7 on a D845GRG motherboard with an Intel Pentium 4 processor at 2.4ghz (not sure if this last bit is relevant)

The dual monitors worked fine on initial startup (and still do) with EFI strings from OSX86tools (as well as EFIHEX). I've found that I can run NVinject as well, (on its own, without the original strings) but I get the same exact artifacts issue. I've attempted NvDarwin for both 512mb and 256mb but neither have been able to get the card running at all. The same for NVkush. I've installed drivers from the 9f23 installer and from the Nvidia Universal installer, which have allowed me to use the EFI strings. Booting with only a single monitor on both the DVI and the VGA outputs of the card also still causes the artifacts. I've been working on this issue for 2 straight weeks now, and am at the end of the proverbial rope, desperate enough to start asking the internet for help.

I'm kind of hoping someone has had a similar experience and would at least be able to identify what the term for these artifacts is for further searches. or (hopefully) suggest some kind of a solution. Thanks a ton for any help.